Common fillers for resin casting

It is common practice to add fillers to resin for a variety of reasons, for example .. to reduce cost; to alter the strength, weight or appearance of the cast; to make the resin more workable either before or after hardening. On the whole almost anything which is completely dry and inert (i.e. is not and will not become chemically active) can be used as a filler in both polyurethane and polyester resins. The Part A ‘resin’ component of most 2-part polyurethane resins comes with a certain amount of filler already included. I cannot say anything specifically regarding epoxy resins since I don’t work with these myself, but most of the following fillers would also seem to be suitable according to Tomps and other sources.

Amongst a wide range commonly used are Fillite (an alumino-silicate ash) and other ‘glass microspheres’; chalk, talc, marble dust or limestone sand (i.e. different versions of calcium carbonate); sawdust, chopped glass fibre, powdered slate and ground olive stones. Some, such as Fillite and glass bubbles, will reduce the weight of castings considerably and also make the resin more carvable. Others such as glass fibre and marble dust will make the resin stronger. Fillers are also invaluable when slush casting with resin (building up a hollow shell) because the resin will automatically lay down a thicker coating. There are a number of metal powders normally used with polyester resin which give castings the look and some of the properties of real metal, including the weight if enough is used. These are not normally used in polyurethane resin because the opacity will dull the effect. The same is true for other fillers meant to impart a surface appearance such as powdered marble or Portland stone, or sawdust .. with these polyester resin will usually give a better result.

Apparently the presence of a filler in resin will subdue the exothermic heat build-up, consequently slowing the reaction down which reduces the shrinkage rate. This makes sense but I haven’t noticed much difference myself because I most often work with quite small amounts. The reduction in shrinkage also stands to reason, but more so because in the total mix there’s less resin to shrink. When working with polyester resin it is often recommended that if a substantial amount of filler is used the percentage of catalyst added should be increased from the usual 1% to at least 2% (even up to 4% if mixing small volumes) in order to compensate for this and speed up the setting.

As a general rule, the finer the filler powder the quicker it will thicken the mix i.e. the resin could lose much of it’s free-flowing property even at just 10% addition of the filler. This is the case for example when using ordinary talc. But many fillers such as Fillite and the various ‘glass bubble’ types have been developed with smooth or spherical particles, which means that they affect the flow and viscosity of resin much less. Some can be added as much as 100% by weight to resin without making it any less pourable.

Amongst suppliers and practitioners alike, opinions seem to differ as to whether or not resin should be catalysed before mixing in the filler. What works best will often differ according to the type of resin, the type of filler and the way the mixture is used. I would say as a general rule again that if the filler is going to alter the viscosity of the resin substantially it should always be catalysed first, otherwise the risk is that the catalyst can’t be properly dispersed through the mix afterwards. This is particularly the case when adding a filler which has some absorbency such as sawdust. If absorbed into Part A first the risk is that Part B may not have a chance to combine with it all. An exception is Cabosil when this is allowed to soak in resin overnight before using. Catalysing first and then mixing in filler will of course shorten the amount of time one has to work with the mixture.

I’ve shown, for example in my post Filling polyurethane resin with used tea or coffee from May 1 2013 or in the article Using plaster as a filler in this section, that choices of filler aren’t limited to those recommended or marketed. If you have any other ideas you’ll just have to test first .. and be prepared for unusual behaviour! .. and if the results are interesting please tell me about them! Remember though that as a general rule resin will not like anything containing moisture or active chemicals.

Many of the fillers in the following list are also suitable for mixing with plaster, provided they are non-absorbent and don’t interfere with the plaster’s water uptake. For example I know that Fillite works well with a hard casting plaster such as Crystacal R and I’m assuming the same for Noblite or glass microspheres.

I have adjusted prices to include VAT as I always do and the last update was May 2016. Those included don’t necessarily represent the cheapest, but are there to show how the price varies .. greatly in some cases!

Does adding a filler make resin stronger?

I am dealing with this question separately because it’s one I get often. The straightest answer is .. no it doesn’t .. not as a rule, in fact it’s usually the opposite. If you read the information for the various fillers offered on suppliers’ websites you’ll see reference made to using fillers to cut costs, alter appearance, alter weight, to make thicker etc. .. strengthening the resin is low on the list if it’s there at all!

Of the most commonly used fillers .. at a guess Fillite, glass microspheres, talc and marble dust .. only the last two can improve strength (impact resistance or surface hardness) when added in the right ratio and then it depends on which resin is being used. For example I do know that talc will make polyester resin less brittle (since polyester suffers from this) but I doubt whether it will add much extra to epoxy and I’m pretty sure that in the case of polyurethane resin it will weaken it, just as other fillers do. Another  of the main reasons for adding fillers is to make the cast material easier to machine or sand, in other words basically weakening it, and this is particularly true of polyurethane.

Normally I would say that if you’re worried about structural strength you certainly shouldn’t be using either polyurethane or polyester with any powder filler .. you should either use epoxy resin, or think in terms of fibre reinforcements or fibrous fillers. If it’s impossible to include a matting layer i.e. glass fibre within the cast, that’s where these fibre fillers come in handy. I’ve included some of them at the end of the following list.


an industrial ash by-product, more specifically ‘microspheres’ of alumino-silicate. Because the particles are hollow the material is very light and because they’re spherical resin will remain fairly pourable even with a high proportion of Fillite. Overall light grey in colour but with a speckled appearance. Commonly used in the ratio 1:2 Fillite to resin by weight, in which the resin remains freely pourable but almost 3 times as much can be used making the mix more like a filler paste. Standard precautions against airborne dust should be taken to avoid eye irritation. Although prolonged breathing of airborne crystalline silica is thought to be a possible cause of silicosis, its form as Fillite microspheres is considered harmless according to the official MSDS.

Fillite used to be very economical price-wise but this has rocketed in recent years (e.g. now recommend using the alternative Noblite).

Obtainable £4.79 per 1kg, £16.50 per 5kg; £5.82 per 1kg, £60.22 per 20kg

Noblite microspheres

Noblite has similar properties to Fillite, but minute bubbles of glass. Very light-weight and flow-assisting. ‘Noblite’ is just one brand name but there are many others. Glass bubbles or ‘microspheres’ are suitable for all resin types and will thicken, reduce weight and make casts easier to sand. recommends maximum addition of 20% by weight for their very light-weight ‘microspheres’. As you can see below, prices vary a lot according to quality or specific grade

Obtainable £26.40 per 8kg; ‘glass microspheres’ £4.74 per 100g, £26.34 per 1kg

Calcium carbonate

All of the following are basically just different versions of calcium carbonate, either naturally occuring and processed, or manufactured.


or talcum powder, also known as ‘French chalk’. There are different grades i.e. AT200 talc ( is especially fine and serves particularly well as a filler if the resin needs to be machined after casting, making it firm but easy to sand. When added to resin fine talc will thicken it quickly, changing the viscosity noticeably even at just 10%. Talcs will also generally improve brittleness (meaning that they’ll make polyester resin less brittle, more impact resistant) and make the surface more durable. Since normal talc thickens resin quickly it is excellent for making a filler paste (such as normally bought for car body filling) even with polyurethane resin, although working time in this case is very brief!

Some types however are specially coated to counteract the immediate thickening effect. Omya BL-OG, for example, will affect resin viscosity less and can be added up to 40% before any major change, according to Tomps.

Obtainable AT200 £6.40 per 25kg;  Omya BL-OG £5.04 per 1kg, £29.88 per 10kg, French chalk £5.06 per 1kg

Limestone sand

This differs from regular sand in that it’s pure limestone (calcium carbonate) with no silica which means that casts can be more easily tooled afterwards. Trucal No.4 has a particle size like table-salt and also serves as play sand in children’s sand pits.

Obtainable Trucal No.4 £5.76 per 25kg

Marble dust

Will not only add strength and weight to resin but also a stone-like coolness when the proportion is high. Marble is commonly available in various particle sizes and the coarser versions can also reproduce some of the surface sparkle of carved marble.

Obtainable ‘marble flour’ £5.05 per 1kg, £21.40 per 10kg; ‘marble powder’ £4.51 per 2.5kg, £18.38 per 25kg; ‘marble filler coarse’ £4.56 per 2.5kg, £18.89 per 25kg.


also calcium carbonate but finer than the above. I still don’t understand where the difference is between this and ‘talc’ though it is probably coarser

Obtainable £9.36 per 25kg

Slate powder

Finely powdered slate, gives a dark grey slate finish to castings and is also fairly cheap for general purpose filling.

Obtainable £4.90 per 2.5kg, £21.35 per 25kg


is ‘fumed silica’ and one of the more expensive fillers. In a sense it is more a thickening ‘additive’ than a filler, since only very small amounts are needed to turn polyester resin non-slump or gel-like. The Tomps website recommends soaking the Cabosil in resin overnight (before being catalyzed of course) because it can be difficult to disperse and the limit should be 15% by weight. Being silica-based it will also fill and bond with silicone rubbers. sell their own ‘fumed silica’ and recommend adding just 0.7-1% by weight to make a non-slump liquid, 2-3% to make gel-like or slightly more than 3% to make a filler paste.

Obtainable Cabosil £6.73 per 250g, £28.08 per 1.25kg ; fumed silica £4.74 per 45g, £16.74 per 450g

Aluminium trihydroxide

can be added to all resins to a 200% by weight maximum (according to although at this ratio the resin will be harder to pour. It will add rather than reduce weight but, more specially, will reduce shrinkage and also add fire retardancy. Tiranti markets it as ‘synthetic onyx’ saying that it gives ‘excellent translucent qualities similar to onyx and some marbles’ when used with polyester resin.

Obtainable £4.74 per 1kg, £29.94 per 10kg; £9.95 per 2.5kg, £53.65 per 22kg

Metal powder

This is real metal, but very finely ground into a powder. Brass, copper, aluminium, iron and bronze are commonly available but nickel can also be found. Brass powder particles can be spherical if produced by sintering. Normally metal powders are only used with clear or transparent resins. Iron powder may just serve to increase the weight of casts (rather than any look it imparts) and can also be added to plaster to give a special effect as it rusts. If all that’s needed is a metal effect on the surface it’s common practice to fill a small amount of catalysed resin richly with the metal powder (Tiranti recommends the same volume plus a little more) and apply a thin coat to the mould surface first before filling the rest with unfilled or pigmented resin. Once cured properly the surface of the casting needs to be cut back i.e. polished with steel wool to expose the metal particles and achieve a metallic shine. Filling a large cast completely with metal-filled resin would be very expensive!

Obtainable bronze £22.20 per 1kg, iron £13.14 per 1kg; most metals £11.99 per 500g, inc. iron

Fibre fillers

Fibre or ‘fibrous’ fillers are principally for adding strength, as is particularly the case with glass-fibre strands, but the finer ones can also serve as thickeners when mixing.

Milled glass fibres

Suitable for adding to any resin to ‘improve mechanical properties’ which means making stronger, in some cases more weather-resistant and can also act as a thickener. Milling produces very short glass filaments, rather like flock. If used as a strengthener only a small amount may be needed as long as this is distributed properly.

Obtainable £7.14 per 500g, £47.40 per 5kg

Chopped glass strands

The larger-scale version of the above, as it were .. the individual strands of glass-fibre matting cut very short. The options here .. dependent on the length of the strands or the type of mould .. are either to mix directly into the catalysed resin then pour, or to apply a coat of resin then sprinkle on top.

Obtainable £3.66 per 500g

Milled carbon fibre

Very short fibre length .. appearance almost like a powder .. but will strengthen resins much more effectively than any granular filler. Excerpt from product entry on EasyComposites website:

“.. adding around 10% by weight milled carbon fibres to a traditional FastCast Polyurethane Casting Resin will transform an often too-brittle plastic into a far stronger material with increased tensile strength (snap/breaking strength) and durability. Adding the milled fibres to epoxy casting resin can improve an already high performance cast plastic into an incredibly strong composite with mechanical properties to rival the toughest thermoplastics for strength and durability.”

Obtainable £14.70 per 400g, £47.40 per 2kg

128 thoughts on “Common fillers for resin casting

  1. I was planning on making resin masks and got something called perlite filler would this be good stuff to use it seems wrong as it is like little beads and not powder and there are no instructions on how to use it with the resin

    • I haven’t heard of this, but ‘beads’ is not wrong .. many fillers are micro-beads because this still allows liquid to flow. Re directions .. you’ll just have to put in a little bit of time searching online for the product .. there must be something!

  2. Can you add cement as a filler? I am hoping to make durable terrain for miniature wargaming. Resin gives me the ability to replicate my designs quickly but the resin on its own is too brittle. Weight isn’t really a factor as the terrain pieces aren’t huge themselves.

    • Hello Michael,

      This is a question I’ve had before and, though I’m not enough of an expert to be really sure, for the moment I have no say ‘No, this isn’t going to help at all!’. For example .. cement is very strong when mixed and cured as it’s supposed to but when added to resin it just becomes like any other powder. I can’t even vouch that it can be used as a filler without affecting the proper curing of the resin itself. After all, fillers for resin really should be inert, and cement is definitely not! The only additions to resin which will strengthen the cast in the way that you want are the ‘matrix’ kind i.e. as in fibreglassing, including a layer or two of binding mesh (glass fibre matting, jute scrim or similar) when casting. At least, that’s what I’ve always assumed up to now. I need to look into this though and deal with it a bit better in this article. You haven’t said which resin you’re using .. polyurethane, polyester or epoxy?

      • I’m using polyurethane resin, I gave the cement a try but it didn’t work that we’ll…. Wish I waited for you’re comment before trying.

        I also gave very fine sand a try, it made the test piece strong and less brittle but the weight of the sand pulled down so the sand was visibly denser at the bottom of the mould.

        I’ve found a fibreglass fibre filler supplier, does this sound like the stuff I need to fulfil my intended needs?

    • Have a look at new info I’ve put in the article, particularly re ‘Milled carbon fibre’ at the end of the list.

  3. Hello, I am wondering if you could give me some advice. I am casting a product which is quite an intricate design I have been using polyurethane resin, I have been adding fillite because of the thickness of the piece. I have used it at 200g polyurethane to 100g fillite. but I have been getting bubbles in the product. I have reduced the ratio to 220g polyurethane to 40g fillite but I am still getting bubbles even though I am using a vibrating machine. I was wondering if you think it might be possible to use 100% of polyurethane up to 5mm in the mould and let that set and then file the rest with a polyurethane and fillite mix.
    My main worry is that the two won’t bond together and may separate over time.
    I would be grateful for your advise on this subject or maybe you have a better solution to my problem. Thanks Catherine

    • Hello Catherine,

      No there definitely won’t .. certainly shouldn’t! .. be any reason why the straight resin doesn’t bond with the resin/Fillite mix so yes, that sounds like a good solution! The bubbles may be coming because your Fillite has absorbed some atmospheric moisture (container left open, not stored tightly enough ..?) and the resin is reacting a bit, producing carbon dioxide.

  4. Hi David,

    Amazing information on casting for the new comers like me.
    I want some advice from you on casting multiple cavities in polyester resin mixed with silica powder and ratio is 3:1 where 3 parts of silica powder mixed with 1 part of polyester resin.
    I tried but the liquid is like paste form after mixing the silica and it’s difficult to pour the material in each cavity. The cavities are of cylinder shape and height is about 10 mm and weight is about 8 grams each.

    Can you suggest me the process that how to properly mix and pour and cast this material? I am also looking for some semi automatic machinery for production run. Can you suggest me some idea and guide me.

    Best Regards,
    Bhavin Doshi

    • Hello Bhavin,

      I would normally say that the only solution would be to use less silica powder, surely? I haven’t used this myself. I’m publishing your query in case anyone else might be able to advise .. for example Will Huntley, who occasionally replies to comments here, would probably know more.

      • Good Afternoon Bhavin and David,
        Yes, I would use less silica powder and/or replace it with another product such as Cabosil, Cenospheres, Qcels, or Talc . Each will give a different result as far as sanding, strength, flowability, and weight. At the end of the day it all depends on what you want to object to do, feel like, weigh and/or cost. Cabosil, is good for making gelcoats, Cenos (aka Pozospheres) and Qcels help with flow, Talc is a good cheap filler. Qcels also sand easier. Hope this helps🙂

  5. Hi Fabulous David,

    i am using Polyester resin to make cartoon characters.

    usually they are attractive with many colours. i want to avoid painting after the casting rather to cast all the small parts i.e eyes, teeth, magic wands, blushing cheeks etc with pigmented polyester resins and attaching with a glue(which type)

    what would you do?

      • Re: Fabulous David -Polyester Adhesives. I have used many different this to glue fibreglass together. Araldite 2014-1, Araldite 2015, polyester and vinylester (aka Derakane) and standard laminating epoxy. With Polyester and vinylester I would always brush a light coat of acetone over the cured fibreglass to “activate” or prime it. When the acetone dries it should be tacky and the you may apply more polyester or vinylester. It should bond fine but if you put on too much or do not do the priming…you may get separation. A stronger bond would be If you are using an epoxy…simply rough the existing surface of fibreglass and then apply the epoxy. .
        Hope this helps☺

  6. Hello David,
    Thank you for a great list you provided us with. My question is what sort of texture and colour effect (when dyes area added) result when fillers are added. I am looking to obtain a matte and opaque quality to the resin. Do the fillers create that effect or is there something else that is required in process?

    • Yes, probably any filler with make the resin opaque .. but finely ground ones in particular. But as far as I know a filler won’t make the resin surface matte .. the surface exposed to the air will still be shiny, at least I know it always is when using polyurethane resin. Polyester resin, on the other hand is generally not as shiny. But also, as far as surface quality goes, this is more dependant on the mould surface it’s cast into isn’t it? For example, if you make a silicone mould around a lightbulb, the silicone will reproduce the ‘polished’ surface of the glass and transfer this in turn to the cast, whereas a prototype with a rougher surface will result in a ‘matte’ look on the cast surface. Either this, or you have to abrade the cast surface afterwards to matte it.

      • Hi David,

        I found your site by coincidence, interesting reading. Do you have any experience with Acrylic systems? My company makes PMMA (Acrylic) systems actually as special fast acting temperature tolerant binding systems for highways, but the same polymer system is used for water clear resin casting with complimentary PMMA microspheres. Would you be interested to try some? We also have a range of glass powders that also might be suitable as a functional fille rfor your resins systems

  7. David,

    You may have answered this question already in a comment above, but I want to clarify because I was a little confused by the word choice of your reader’s inquiry.

    My issue is that I am trying to pour a crystal clear resin over rough barn wood to be used as a counter top. The wood has some holes, gaps, dips, splinters in the wood, etc. I want to see all of the rough wood underneath, but hope to have a thick layer of clear resin so as to have a flat, food grade surface that will not give my guests or myself splinters. I have never used epoxy resin before. The bar wood is glued and nailed to the counter tops, so it is not perfectly level and I have no way of leveling it (if I sand it down it will ruin the look I want). I figured if I could find a way to thicken the resin, it would move slow enough that it would stay mostly on the surface and remain thicker in depth.

    Is it possible to thicken a clear resin? Say from a normal viscosity to a more syrupy viscosity?

    If not, is it possible to let a layer cure, and add another layer of clear epoxy to build up the surface to a desired depth while maintaining a crystal clear view of the barn wood below (not seeing “layers” of epoxy but looking like one deep, clear coat)?

    Please help.


    • Hello Joshua,

      There are self-levelling crystal clear epoxy resins for this very purpose .. but at the moment I can’t remember where on the net I’ve seen them. Maybe you’ll find them if you google ‘epoxy resin for encapsulation’ or something similar.

      • Thank you for the reply. I purchased 4 gallons (to cover roughly 30 square feet) off of this site because it boasts a 1/8 in thick self-leveling flood coat, where as everything at Home Depot or other hardware stores listed 1/16 in thick coat, or “50 layers of varnish”:
        and it seems to indicate that I can complete layers if I pour, wait 4 hours, then pour again, but I don’t know if this will dry clear. I considered doing a “test pour, but the mixing steps are very specific, and I don’t want to waste the resin because it is quite expensive. Do you have any experience with this product, or something like it?

      • No, I haven’t worked very much with epoxy resin. All that I know for sure about epoxy is in my page on ‘Epoxy Resin’ in the Materials section.

    • GDay Joshua,
      I have considerable experience with a product from the US called Kraft Kote. It is a two part epoxy that i have had many clients use for this specific process. But please note a couple of things. 1. This is a self-leveling epoxy…so if the top is not will have a nice glossy puddle on the floor. 2. I have poured this product to a thickness of about 1/8th to 1/2 an inch. But if you want thicker, you can may pour it in layers by letting each dry and then pouring another the following day. 3. “FOOD GRADE” with regards to plastics, usually means that they can handle some heat without failing, melting, burning, cracking, etc. Kraft Kote is NOT food grade. Nor are alot of things you use in the kitchen. But, it will make your bench gorgeous…just use trivets and don’t put a hot pan directly on it.
      All the best, Will Huntley

  8. Hi Guys ,
    I’m very new to this, and Im sorry for my newbiness🙂
    I want to make a two tiles for my bathroom, and I don’t know about which type of casting matrial should I use ? I want it hard and clear not brittle almost like a hard glass tile.. and I don’t know which one to use epoxy ,polyester , or polyurethane ? please help

    • I’d recommend you use clear epoxy. This is tougher than polyester, not as brittle .. and I don’t consider clear polyurethane an option for home use because of the health issues.

      • thank You David for Reply,
        I will try epoxy , but I epoxy is yellowish after the cure ? i want it to be no colored clear , can it be done with epoxy ?.

  9. Hello David!
    Congratulations on your website, it was amazing to find all the information you have shared with the public.

    I am writing from Brasil to kindly ask for help: I am trying to make a marble dust surface finishing to cover the surface of an open air art sculpture made of concrete plates. The idea is to apply a thick layer of marble dust pre-mixed with epoxi resin, spreading it over the concrete plates surface, and once the resin is cured and dry, to grind it and polish. The tricky thing is that when I mix the marble dust with resin, the marble dust turns grey! I have been using titanium dioxide powder (white pigment) to make it white again, but i feel it comes out less natural then if i was able to preserve the marble dust natural color. I am making several tests trying to make the powder impermeable before mixing it with resin. My impression is that the reason the marble dust turns out darker (more grey than white) it´s because it gets ‘wet’-like, soaked in resin, just like beach sand turns darker when wet. Please help me figure this out, how can i preserve the white tone of the original marble dust when mixing it with resin (without using titanium dioxide or any other pigment)?
    Sorry for the long explanation and thank you tons in advance for the attention.
    Best, Marcello

    • Hello Marcello,
      I haven’t used marble dust as a filler that much myself but this sounds like it could be a common problem. My instinct is that it may unfortunately be due to the ‘quality’ of the marble dust you’ve been sold i.e. cut with other things or not derived from marble at all. It may be that using a slightly coarser-ground version might help because this is likely to be more opaque. I’m sure that adding a careful amount of pigment is a common solution .. but perhaps you should try a more translucent alternative to titanium white instead? .. i.e. what we call here ‘lithopone’ or even just ‘whiting’ .. I know that titanium white tends to blanket too much. May be a silly question but .. it’s not the epoxy itself that’s dulling the effect? .. have you seen its cured colour on its own?
      best, David

  10. Hello David,

    My goodness what an excellent and informative discussion on the properties of fillers. I have a question which I hope you can answer. I am casting a small electricity generating turbine in polyester resin, which will be exposed to the elements. There will be voltage, magnetic fields and heat to take into consideration. I would like to add a filler that will allow an hour or so of working, have a pourable viscosity, maximise heat dispersion and not affect the magnetic and electrical properties of the generator. I guess, unless you know different, a non-ferrous metal filler like copper filings would be required but what would be the effect on the other characteristics I require and how much filler would I be able to add per litre of resin?

    • Copper .. or brass .. powder I would think. But there’s no polyester resin giving an hour’s working time as far as I know .. do you know of one? If so .. what, where? It’s usually max 15-18mins. Neither copper nor brass powder will affect standard working time too much. Metal powders in polyester resin will be affected by ‘the elements’ .. both will turn verdigris. Is this the intention?

      • Thanks for your reply David. I will have to modify my working method/plan to take into consideration the short pot life. I’m not too bothered about discolouration caused by oxidisation of the copper or brass filler however the fact that it does oxidise when encased in polyester resin gives me some concern that the casting may not be impervious to air. So now the problem seems to be that I can reduce the heat build up when the generator is running with a metallic filler but a coat or two of paint for sealing will reduce the heat dissipation.

      • No, actually this may not be a problem because in order to give the oxidised effect when used in sculpture the surface must be abraded to expose the metal particles. I’m assuming that if not abraded the surface would be secure, but I can’t vouch totally.

  11. David, good site with lots of useful info
    Do you know of a filler that would thicken either polyester resin or epoxy, yet be clear? I am also wanting to increase the density. Even though micro-sphere glass will lessen the density I tried it just to see if it would become clear in the resin, nope, it makes the resin white/opaque. I tried ground glass and got the same effect. I am thinking of trying 1/2 chop fiberglass, but, i know that won’t keep it crystal clear, just not sure how translucent it will get? I am thinking of trying broken glass, but, i have seen broken glass caste in clear epoxy and it looks like broken glass. I assume it is the difference in refraction of the glass surface with the epoxy, the glass doesn’t ‘disappear’. I will may mix some broken glass in for an effect, but, i would like to make a clear epoxy paste or putty. Any ideas? I have seen in the US one company advertises a clear epoxy putty. Haven’t ordered it yet, but, it is sold in a 4 0z package and is way too expensive for my needs. I need to make a kg at a time and keep the cost down.

    • Yes James, considering the efforts you’ve gone to already I’m surprised you haven’t heard of this .. thixotropic additive! It’s not uncommon when working with polyester, and although I didn’t come across any when researching epoxy resin I’m sure there must be one. The one I use from Tiranti looks opaque but this disperses through the resin keeping it relatively clear. It will not increase the density though .. I don’t think there’s anything that will while keeping it transparent! Here’s the brief entry in my ‘Polyester’ article in materials:
      ‘Thixotropic paste (Tiranti) not a separate chemical agent but a gelled form of polyester resin which is ‘100% thixotropic’ and which can be added to other polyester resins to thicken them. Can be used as a resin by itself if catalysed. £11.92 per 1kg inc. VAT 2015.’

  12. Hi David,

    I want to create a bio plastic which is flexible.
    I’m experimenting with Rosin. It’s resin from the pine tree. When I cast it it’s very brittle. I’ve already tried to add beeswax, oil and charcoal to make it less brittle and more flexible like plastic.

    Do you maybe have any tips?

    Thank you very much.

    Lilian van Daal

    • I’m afraid I don’t at the moment. Have you looked for info re ‘Bio Resin’? .. it’s similar to what you’re trying.

    • Hello David,

      I was considering making an alternative to plastics that have similar properties but would possess the added advantage of biodegradability. I was considering using rosin and beeswax, and it seems that some people who are working with rosin add charcoal to this mixture. I’ve also seen bioplastics based on corn starch, glycerin, vinegar, and water. I was wondering if you could give any insight as to how to go about making this material (i.e molds, components, etc.) Additionally, I was wondering (since you don’t make email addresses public) could you forward this query to Mr. Van Daal?


      • Sorry, no insight, I know nothing about this .. although a potentially interesting subject! Most of my experiences with commercially available Bioresin have been bad though .. i.e. failing to cure, remaining soft, ‘weeping’ in places, smelling more disagreeable than the less ‘friendly’ kind!

  13. Hi David, I was hoping you could help. I’m trying to cast some miniature stone walls made of clay to replicate stone and wondered if there was a ready made product that would do it. I’ve tried adding all manner of things to resin (sand etc) but there’s too much frothing even when I’ve dried it for 24 hours in an oven! Ideally I want the end product to be fairly robust. Thoughts?

    • This shouldn’t be happening if you’re taking those steps! Could you tell me exactly which resin you’re using, what you’re using for the mould, where the sand comes from etc?

  14. hello there Do you know of a type of additive that is reflective..or can enhance light? to add to cast silicon or urethene objects, I.e. I am using led lighting and wanting to spread the light into my castings??

    • Interesting! There may be some effect if you add mica particles (this is the reflective ingredient in ‘metallic’ paints) and I’m pretty sure this is obtainable. Otherwise or in addition anyway it’s worth looking at MindsetsOnline (just google the name) because what you’re seeking sounds a little like the kind of ‘smart’ material that they specialise in. By the way, you may achieve it with polyester resin, especially the clear-casting, but not the regular polyurethane since this is opaque .. and I’d be very cautious, for health reasons, about using transparent polyurethane!

  15. Hi David,

    I tried plaster of Paris as a filler and it makes the polyurethane resin heavier and gives an effect that looks like water-fountain statues. No bubbles either for I let it sit an hour in the A part. Semi glossy too. However, when this mix is rotocasted, the results can be fragile and if accidentally dropped, it will shatter like glass.

    I am experimenting with hydraulic cement into the resin now. It sinks down. But it makes the resin tough. Almost like hard vinyl. I rotocasted a few figures and the feel is like vinyl. The figures can be sqeezed moderately and there is flexibility. Still experimenting with this. Keep you posted.

    • Many thanks for this Ralph! I’m surprised .. I mean, in a positive way! .. there’s potential here perhaps! I’ve often thought of trying the same with the various plasters and cements I’ve got, but haven’t got around to it. I forgot to say in the last reply but I thought afterwards there may also be a problem re the PU foaming because of slight water content in the plaster .. it being hygroscopic? The ‘baby powder’ presumably talc, is more standard and would I think assist the rotocasting method more .. but, yes, baby powder is a rather expensive version of something that should be around £1 per kilo.

      This is a weird result with the cement though .. it does sound as if the resin isn’t curing fully i.e. stuck in the ‘green stage’ because of something in the cement? I think you’ll know for sure if the casts start ‘sweating’ or bleeding liquid. While you’re at it .. have you tried flour .. or icing sugar?



  16. Hello David
    Thank you for an interesting topic, I am building a 42ft wood epoxy sailing catamaran, I have access to waste(off cuts), extruded polystyrene which I thought might make a good light weight filler which could be mixed with epoxy in a relatively high ratio/ volume to make very lightweight easily sand-able filler. I would need to reduce it to dust somehow.
    What do you think?

    • No, I’m guessing this would not work .. that is, I know it wouldn’t work with polyester resin because the powdered Styrofoam would dissolve in it and no doubt affect the setting of the resin in the process. It could be the same with epoxy resin .. but do a test .. let me know the result!

  17. Hi David,

    I am casting a few copies of my sculpt out of polyurethane resin and rotocasting the pieces. Would Plaster of Paris make a fine filler for sanding and strength? I would use marble dust but its impossible to find on this small island and no hobby store on mainland will ship it here. Baby powder is my only alternative for now. Please help.

    Thank you for your time,


    • I’ve been asked this a few times and I think that, plaster would not make a good filler because it is not inert. I think it could affect the resin .. but do a test and let me know!

  18. Hi David!
    I’m working on a research project for school, and am trying to create an alternative filler for the polyester resin. I have the idea of using collagen from fish scales / bones; is this possible? Please let me know as soon as possible. Thank you!

    • Hello Emily,

      Is collagen simple and cheap to produce? .. or is it a common by-product? .. otherwise it may not be viable as a filler. Also, fillers need to be chemically ‘inert’, not affecting the chemical reactions in the curing resin, and I’m not sure that collagen would be. I’d be interested to know why you’ve chosen this?

  19. David,
    Thank you for this fantastic source of information!
    I make faux bois furniture and lighting from concrete/ferrocement. I’m interested in producing the lighting, and perhaps other items, using the casting method rather than hand sculpting. The resulting castings would have to hold up outdoors/UV stable, and be very strong as they will be representing my craftsmanship as retail merchandise. And, they will have to mimic the look and feel of concrete, although for the lighting I’m not opposed to a reduction in weight.
    What type and % of filler could approximate a nice, dull, gray finish with a cool touch? What type of resin to use? Would you include fiber, and if so, which type? Which type of mold rubber to use? Will resin hold up to the weather and sun? I’ve seen furniture and other objects made from “resin” and I’m impressed with the results. Thank you for your help!


    • Hello Mike,
      Just a question, or two, first .. why are you considering faking ‘concrete’ or cement look for producing articles meant to survive outside? Why not keep with the cement-based media, since these is designed for survival outdoors? Do you use an acrylic polymer admixture with your cement? It greatly increases the strength and weather-resistance and, if mixed in a certain proportion with cement, results in a sculpting mass very like natural clay .. though one has to model comparatively quickly with it? The polymer will also help it to stick to steel reinforcement structures.

    • At Dryden Aqua we process 40,000 tonnes of glass per into fine high grade powders from 10 microns up to 1000 microns. For filler in resins we normally recommend, 1 to 50 micron our our 10 to 250 micron product.

      Cost is £0.5 / kg supplied in 25 kg sacks, minimum 1000 kg. We usually supply at 24 tonne lorry loads at £0.25/kg.

      We can make any particle size to suit an application as long as it is in sufficient quantity.

      • I don’t usually endorse spam or self-advertisement .. but thought this may just about fall into the ‘useful info’ category, even though I sincerely doubt that anyone reading this here would need a thousand kilos!

  20. Can you please suggest the best chemical additives for polyester resin body filler nd their concentrations

    • I looked into this a while ago and got quite a good result using a combination of GP (general purpose) polyester resin, thixotropic polyester paste and either talc or chalk dust. .. and of course MEKP catalyst. The concentrations of the main three really depend on the consistency you want .. you can juggle them, you’ll just have to experiment.

  21. Is there a way to ad a textured feel to the cast resin ? or does this texture have to be incorporated in the mold already. I need to make a grip for something but i think that when i will be molding and casting it in a regular fashion it will come out very shiny and slippery in the hand.

    • You’re right .. the texture would have to be in the mould. The only other option would be to cast with a regular and smooth surface and then make a coating mixture with the same resin and coarse sand or grit and apply this to the cast surface, but this is a lot of extra work and it would be very difficult to apply this uniformly!

  22. Hi David,

    Can you please tell me the procedure of adding Talc to manufacture polypropylene? What should be the percentage? Do I have to use any additives with that?

    • I like the wild questions! .. they force me to check up in case I’ve missed something! I can’t give you an answer because polypropylene is an ‘industrial’ plastic, a thermoplastic which needs industrial-scale heat and pressure to work with it. It’s only available to individuals like myself in ready-made sheet form. Actually working with it in melted form is beyond my means.

  23. Hello
    Devid Sirand & Dr. Preeti Gupta
    I am working on Unsaturated polyester resin based capsules for under ground construction (Mines & Tunnels). There is no problem in setting time and mechanical properties of the resin capsule. I working on Double chamber base capsule and glass tube based resin capsule. It is temperature sensitive product but we Adjust the setting time with accelerator not by catalyst. my special product’ s CS (MPa) is 100 within an hour and setting time 30 sec to 300 seconds. I am using barytes powder and few special fillers. Please contact to me on
    Dr. Please give your contact details, I will try to short out your problems

    Manoj Kumar

  24. Hello David,
    Can you please tell what type of fillers can i use to make casting grade polyester resin cheaper. Also it should not affect the pearl effect in the product.

    • Hello Ashok,

      You’ll need to explain a bit more I’m afraid. By ‘casting grade’ do you mean ‘clear casting’ polyester? There are only basically two pourable types of polyester resin .. this one, and the less clear, slightly beige-coloured ‘general purpose’ (sometimes called ‘multi-purpose’) commonly used in fibreglassing work. You probably mean the ‘clear casting’, but there’s no ‘pearl’ effect with this as a rule .. it’s as clear and colourless as water. If you mean a filler that doesn’t affect the clarity of the resin .. I don’t think there is one! Even glass microspheres will change the appearance.

  25. Hi David. I am thinking of making some children’s climbing holds using a foam and RTV silicone method as described on . The contributor uses lentils as filler but I’m not sure I’d want to use these as when sanding the back of the hold to a flat surface the lentils would probably become exposed and subsequently absorb moisture (I will be fitting outdoors). I was thinking glass beads might be best but do you have any ideas on what might be a cheaper alternative? The hold will be cast using polyester resin and I would be hoping to obtain a fairly vibrant finish using suitable pigments. Any advice would be greatly appreciated.

    • Hello Dan,

      Thanks for the entertainment! I thought .. now that’s interesting, filling resin with lentils .. and not beyond comprehension, after all I’ve shown that used tea/coffee work very well! But you must have been in a hurry when reading the ‘lentils’ part because your contributor just uses them to estimate volumes, not actually as filler! That sort of solves the query, doesn’t it? You intend to use polyester resin, in place of polyurethane. I agree with the Instructables writer that polyester resin can be more brittle .. but I don’t see why it shouldn’t be fine if you’re careful when bolting the forms in place. The forms can be made almost shatter-proof if you give them an inner filling of resin mixed with chopped glass-fibre strands. I say ‘inner’ though because you don’t want glass strands embedded too close to the outer surface! I think the writer has now learnt that you can’t mix anything water-based into PU resin .. and more so polyester resin! You will need to use either dedicated polyester pigments, dry powder pigment or oil colours.

  26. I would like to know what could be the best filler for polyester resin required for making Resin Capsules. We are using baryte but it is causing problems

    • I’m not familiar with the ‘Resin Capsules’ you speak of? It sounds like it’s out of my area. I know that capsules filled with resin are used in some forms of construction as fixings .. but surely this would not be polyester resin? Could you explain a bit more? .. and what problems?

      • First of all many many Thanks for a prompt response!
        Resin capsules/ cartridges are used in rock bolting during tunneling and mines.
        Basically polyester resin is mixed with accelerator (Dimethyl Aniline) and suitable filler. This mixture is enclosed in a separate chamber. In second chamber hardner (Benzoyl per oxide paste) is used. At the time of application both are mixed and bolts are inserted. When the resin cures, it holds the bolt.
        Currently, we are using Baryte powder (200-300 mesh) as filler. Recently we faced a strange and regular problem.
        As soon as the hardner touched the mixture, the resin got cured (and not at desired setting time). We have checked the resin and found it to be alright. The problem is with filler and we are not able to make out, what could be present in the filler that could cause immediate curing and how it could be tested.
        Is it the specific gravity of baryte powder or some contamination? Barite powder is supplied in bags like cement and it is very difficult to check each and every bag.
        I shall be thankful if you can throw some light and guide me regarding the same,
        Thank You very much once again,
        Dr. Preeti Gupta

      • Hello David,

        Any updates after allowing it to dry further. 

        Have my best regards,

        EMEKA NJOKU  (B.Eng., M.Eng.) Managing Director


      • Hello Emeka,

        No. Unfortunately there are definite problems with this. The ‘bleeding’ has continued all this time. It’s not working. There’s something within the ‘black talc’ composition that is inhibiting the proper curing of the resin. It’s not moisture .. the effects would have stopped sooner. Not being a geologist or a chemist, I can’t tell you what it is or how to refine it. However, I still want to do a test using polyester resin (the previous was polyurethane) but I have to do this in my outside space and it’s too darn cold-n-soggy at the moment in London. I’ll let you know.



      • Hi David,
        Can you please throw some light on…what would be the effect of specific gravity of filler on gel time of polyester resin with MEKP?
        I think high density fillers would increase the specific gravity of resin matrix too. But how would it affect the gel time? Also, can further increase in specific gravity of filler may result in settling of filler at the bottom….? I hope you can clear some of my queries,
        Heartiest Thanks,

      • Hello Dr Gupta,

        Luckily Manoj Kumar has replied to your previous enquiry here. He has left his address and I suggest you get in touch with him because his knowledge is more specialised towards your area.


  27. I was curious if I can color my alumilite white polyurethane resin easily with stuff I have in my art supplies already? I don’t want to invest in a lot of colorants. But I have a plethora of craft paints, acrylics, and mica powders. I was going to try some sample batches soon to see if I could get the resin to color but was curious if you might know to save me the trouble of wasting product. I have no problem coloring my easy cast clear epoxy resin with these methods. But I know the resins are different and, as the alumilite is a milky white in color after curing, I wasn’t sure if it could be just as easily colored…

    • I haven’t come across resins from Alumilite here in the UK .. I think they’re just US. Yes, you should test, but it’s safe to go ahead. However! .. nothing which contains water! So water-based acrylic is out .. unless you’re curious to see the resin foaming up and curing like Swiss cheese! Dry powder pigment, or mica powder as you say, or oil paint should be fine! If your resin is marketed as ‘white’ it’s probably pre-filled with an amount of white pigment, so you may not get strong colours.

  28. Hi David,
    I saw some products which made by using resin and powder stone, looks like concrete casting,
    but of course the result is better than concrete casting. I need to create a product and I like the texture of concrete, I’d like to use resin because of having the possibility creating more details without flaws.
    Can I just use concrete powder as filler? What would be the ratio?

    • I don’t know whether you can use cement as a filler. It has chemicals in it which might affect the resin. You should try .. see what happens. As for ratio, any mix which is still smooth enough to pour or spread should be ok. Normally for a concrete look one would use i.e. Fillite and sand, I would guess, or an available stone powder filler i.e. Portland stone.

    • Can I use a mixture of dried clay and bentonite/barite fines as a resin filler. This is a waste product from my thermal drying process for drilling mud.

      Have my best regards,


      Managing Director


      MOBILE PHONE: +2348033099504

      • Hello Emeka,

        I remember you posted the same question last September. I was interested in the ‘black talc’ as you called it then. You agreed to send me a small sample and I sent you my postal address by email. Unfortunately I never received any .. still waiting for that to test!



      • Can you please send me your shipping address. What quantity do I send, 10 kg, 20kg?

        Have my best regards,

          EMEKA NJOKU

        Managing Director


        MOBILE PHONE: +2348033099504

      • Emeka,

        I’ve sent you an email (to the address you’ve included here) with my delivery address. Let me know here if there’s a problem receiving it?



      • I’ve received the email,  though replying to it isn’t working, but I will dispatch the sample on Friday. 

        Have my best regards,

          EMEKA NJOKU

        Managing Director


        MOBILE PHONE: +2348033099504

      • Thanks! The sample has arrived .. thoroughly investigated by Home Office Border Force by the looks of it, but packaging just about holding together. I’ll test it soon!

        all best,

      • Okay thanks. 

        Have my best regards,

          EMEKA NJOKU

        Managing Director


        MOBILE PHONE: +2348033099504

  29. hi
    kindly let me know the process of polyurethane rubber molding in big model like man height..
    first color layer is polyurethane but upper white materials ……..???

    • I can’t add anything to what I’ve already written in ‘Making a supported silicone mould for a life-size head ..’ under ‘Mouldmaking and casting’ in the ‘Methods’ section. Have a look at this first.

  30. thanx for ur reply….can u suggest me what type of polyurethane i will select for slow set curing? …… can i get 10 to 15 mnts pot time? ….so that i can finish the casting.

    • I don’t think you can get as much as 15mins working time with any polyurethane resin. The slowest one I know about is Polycraft FC-6910 which has an extra-long pot-life of 8-11 minutes. You will find it here Another one I’d recommend is Tomps Fast Cast (slow set version) with pot-life of 7-8 minutes. I think generally if you want c. 15mins working time the usual option is polyester resin as opposed to polyurethane. Long pot-life polyurethane resins always have much longer demould times i.e. setting takes 2-4 hours ..!.. with the Polycraft one.

  31. hi Mohamed,
    How to use of fillers in polyurethane resin casting ? mixing process? becoz Polyurethane resin fast curing. so time factor is arises problem in casting time.

    • The answer is to mix the filler with one part of the resin before adding the other part. See my article ‘Making hollow casts in open or ‘closed’ moulds’ which you will find under ‘Mouldmaking and casting’ in the ‘Methods’ section. It explains what you have to do.

  32. I want to use of fly ash “grade -c” as a filler in polyurethane resin casting material. what is the ratio of fly ash and polyurethane resin including hardner ?

  33. Hi David

    Very interesting article. You explain well.

    I am no chemistry student, but I have developed an interest in making figurines as a hobby and also a business.

    I have obtained various molds made from rubber and would like to work with resin mixed with plaster of paris to get the finished product. I would like to get the gun metal finish look for some of them.

    What is your recommendation? Also if I need to paint these objects, what is the best approach to doing so?

    Can electro plating

    • Hello Shiela,

      First of all .. where did you hear about mixing plaster into resin? I’m not aware of this method, and am doubtful whether it will work. Certainly if you want a dark metallic look a common method is to mix a fairly high proportion of real metal powder with the resin and apply this as the first thin coating in the mould. Mixing plaster with this (if it’s advisable at all) would only diminish the metallic effect. Have a look in my ‘Lexicon’ above under ‘cold metal casting’ for an outline of the method.

  34. Hi David
    I am looking for a filler to use with polyester resin metal, specifically to thicken it to a paste without affecting the look. I’ve used a lovely filler, garamite (a mixed mineral thixotrope) in the past but it only comes in large expensive quantities so I’m thinking of trying Fordacal100 (an ultra fine calcium carbonate). Any experience of this, and could it be a good substitute?

    • This I don’t really know .. not having used either of the fillers you mention. Whenever I want to thicken polyester without affecting the look I use a special thixotropic paste from Tiranti which can be finely controlled to achieve any consistency of resin. It’s really good! I’m also wondering whether mica powder (or very small flakes) would be an idea, because this may thicken the resin and possibly enhance the metallic effect at the same time. Mica is used for this purpose in many ‘metallic’ paints.

  35. Hi David

    I want to ask as I was still unclear – when would you recommend to use Fillite vs a hollow glass microspheres? I am not really clear on the difference.

    Thank you


    • I’m not sure that there is that much of a difference, considering them generally .. but I suppose the main difference is that there are a number of brands/types of glass microspheres (with different size and strength) and only one Fillite. So some may impart more strength to a casting, some may sand to a smoother finish, some may even be lighter. I suspect that glass (or plastic) microspheres may be more available and possibly cheaper than Fillite, now that this material seems to be becoming ever more expensive.

  36. Hi there David

    Could I use Talc as a filler / thickener with Polyurethane Resin? I am cold casting bronze statues of about 12″ high and could I add it to the gelcoat or slushcast as well? how much will it affect potlife? As you know slushcasting or gelcoating takes a while, so I need to be sure if Talc will do the job. Also I read elsewhere that some people use Plaster of Paris powder with PU resin for adding weight as well? how will that affect potlife / cast?


    • Hi Martin,

      I only have time for a very brief answer I’m afraid! .. no, not a good idea to add talc to cold-casting gelcoat layer because I’m pretty sure this would dull down the metal effect achievable?

      • Hi David thanks for your answer! so I should just use it as a backfill rather? Also what about my question regarding adding plaster of paris powder to poly resin for weight, would that do the job?

        Many thanks

      • Yes, I think so. I’ve been asked this before .. about using dry plaster as a filler for resin. I simply don’t know for sure because I’ve never tried it myself. My instinct is that it should work because it should stay chemically inert (therefore not affecting the setting of the resin) unless combined with water. But you will have to do a test. But in terms of adding weight, it may be the cheapest but I would have thought there are heavier options, such as marble dust.

  37. Pingback: Top 5 Tips on How to Achieve Maximum Shelf Life and Bubble Free Casts with Urethane Casting Resin! | Raw Material Suppliers

  38. Pingback: Reisn Fillers | brynarichards

  39. Have you ever used cheap casting white plaster or even multi-finish as a filler with polyester resins? If so what were your perceived results/findings?

    Also, I am interested in gaining a solid white marble look from a silicone mould that has a couple of undercuts but mostly has a very easy draw. It is a wall plaque about 2ft x 1.5ft and my initial thoughts are to use clear cast resin mixed with marble dust (coarse and fine) for a gel coat with a little white pigment and maybe some ochre to vary it in places and slush mould it. Presently it is open backed but I will put a back on it to get the section down around the main feature of the plaque which is a horse. The section is about 10mm all round apart from the horse.

    great blog btw.

    • No I haven’t tried casting plaster as a filler, partly because I would assume there may be problems because plaster isn’t chemically inert. It could work though. What do you mean by ‘multi-finish’? If you mean a type of polyfilla then, no this wouldn’t be suitable because it is water-based. Your intentions re marble simulation sound right!

  40. Would be curious to know if you could offer advise on adding filler to food contact products. The resin currently used is PP and of course it has to meet FDA guidelines. Are their any fillers that would strengthen the pp and meet FDA requirements? These are produced using injection molding. Thanks

    • I’m afraid Jeff that you’re asking the wrong person. I assume by PP you mean polypropylene? Do you class this as a resin? It’s not a plastic that can be used in a home or small studio situation .. as far as I know, only industrially .. therefore I can’t offer anything from experience.

  41. Any suggestion on the type and percent of fill to resin when casting a product 3″ thick/12″ diameter? percent of catalyst due to heat build up?

  42. Hi David,

    I am currently carrying out a project investigating how the filler material in polyurethane castable rubber affects its mechanical properties. Predominantly I am looking to find filler materials that will enhance the mechanical properties of a snake drill concept. I will be conducting certain material tests upon differing percentages of the chosen fillers to find the most optimum percentage of filler. The composite structure has to be of a certain hardness as to be hard enough so that it can withstand applied loads yet also exhibit significant flexural properties, not being too rigid and of a certain flexibility, so as to drill in curvilinear directions.

    If you know of any filler materials which you would recommend that would specifically fit these requirements, I would very much appreciate your input.

    Kind regards,

      • How about just any filler materials, that don’t necessarily meet those specific specifications, that you have maybe worked with and you think are worth looking into?

      • Hello Jordan,

        My problem here is that I don’t normally work with polyurethane rubber so I don’t know enough about how it behaves and what is normally combined with it. For example I know that Fillite would be well worth experimenting with if you were using silicone because the silica in it seems to bond strongly with the rubber so that one can increase the hardness in increments without fear of the rubber fracturing. See my post on recycling or extending silicone. I would recommend you try Fillite anyway, and see what happens .. that’s all I can say.

  43. I would be most interested in your test results with the black talc. Would be great if you would post what you have found. Informative site, thanks.

  44. I process waste drilling mud in Nigeria, this is a combination of materials like barite, bentonite, calcium carbonate, base oil etc this is all mixed up with the drilled out soil, mainly clays and some sand.
    The mixture is sent to us after drilling and we process it by running it through a thermal desorber at approximately 650 deg F. The purpose of this is to extract all the liquids (oil and water) the residual ash is my reason for this mail.
    The ash is in the form of a black talc to be disposed. I am searching for a low cost resin to consolidate this ash. Any ideas???

    • Hello Emeka,

      I only have experience myself with two types of resin .. polyurethane and polyester .. although I know that there are various others used industrially. From what you’ve listed, and if oil and water are eliminated, the ash shouldn’t inhibit the setting of either resin. I would recommend trying GP (general purpose) polyester resin because this is by far the cheapest. I’m interested in the possibilities of a ‘black talc’ though! .. so if you’re happy to send me some (i.e a kilo or so) I can at least do some small tests with the resins I have?

      • Hello Mohamed,

        I imagine that either talc, marble dust, chalk etc. could be used to make a polyester paste, possibly also kaolin and even fine sawdust .. though I haven’t made these up myself. I think the polyester would need to be catalysed first as normal before the powder is mixed in, adding in stages until the desired consistency is reached.

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