polyester resin

These page entries are meant to be added to and usually start with general outline information, price guidance, suppliers and useful links followed by my worklog where I can put further info and photos as they come. Details of suppliers are listed in the Suppliers section.


An inexpensive 2-part resin which is usually either translucent or clear, and which readily accepts appropriate colourants or fillers. When cured it is hard, though brittle compared to epoxy or polyurethane unless reinforced.

Polyester resin is available in a number of different forms, the principal being the ‘GP’ (general purpose) resin commonly used for fibreglass work in conjunction with glass matting. GP resin is translucent with a slightly beige/brown tint. The next most familiar is the ‘clear casting’ version which cures glass-clear and colourless and is often used for the embedding of objects in clear blocks. There is also ‘gel coat’ resin which is pre-thickened, and opaque white resins (pre-pigmented) are also common. All are catalysed by mixing a measured amount of the same hardener (containing MEKP methyl ethyl ketone peroxide) ranging from c. 1-4% by weight.

GP polyester resin is often referred to as a ‘laminating’ resin ( i.e. specially for fibreglass work) and the ‘clear’ often referred to as a ‘casting’ resin. Although GP can be used for casting, it is not common to use the clear polyester for laminating partly because it is more expensive.

Advantages of using it

It is very economical compared to other resins (esp. 5kg amounts upwards) /- widely available .. many suppliers and extensive literature/info on the web /-  versatile (different types e.g. general purpose, gelcoat and clear casting, can be made thixotropic) /- very strong (esp. with fibreglass reinforcement .. its most familiar use); UV blocker available (to counteract colour change due to UV light); longer pot-life gives more time for mould-filling or coating (excellent choice for PU foam coating, see ‘Polyester coating’ below); takes powder pigment and oil paint well with good, controllable colour achievable due to transparency; similarly fillers, when used for surface qualities i.e. metal powders, will give full effect; choice of catalyst percentage (standard 1% but more can be added for small volumes); some ‘GP’ versions (such as Tiranti’s) have less styrene emission for better working conditions; uncured resin and work tools easily cleaned up with acetone.

Because it is transparent it is the perfect resin for so-called ‘cold metal casting’ which is the technique of imitating metal by filling resin with finely-ground metal powder and abrading the cast surface to expose the metal particles and buff the surface.

Cold metal casting metal powder £12 per 500g average (tomps.com, Tiranti 2015) Up to 4:1 metal powder to resin (by weight) can be mixed to make surfacing layer with either MP or clear casting resin, catalysed 2% (always add catalyst to resin in this case before mixing in metal. If proper gelcoat resin is used less metal can be added, c.2-3 parts metal by weight. Less than 2:1 is ineffective. Wait until rubber-hard, then fill rest with normal resin catalysed 2% for small forms (1% for larger). This can either be unfilled or if preferred, dark pigmented. Wait at least 72 hrs before ‘cutting back’ and buffing (cutting back is abrading the surface i.e. with steel wool to expose the metal particles properly). Note: Tiranti’s ‘rule of thumb’ is same volume of metal powder to resin plus ‘a little more’ metal powder, and they advise that if measuring by weight the content of metal powder to 1 part resin is; Aluminium 1.25, bronze 6-7, brass 5-6, copper 4-5, iron 6-7


Polyester resin cannot be used indoors without regulation extractor fans and breathing protection because the styrene emissions are harmful, as is the MEKP catalyst. Some polyesters are formulated to have low styrene emission, which may promote a ‘better working environment’ but it doesn’t mean that the same precautions can be ignored!

Strong exothermic reaction may cause cracking in larger volumes (add minimum of catalyst) /- some types e.g. clear casting more prone to surface tackiness (oxidisation) /- brittle if used unsupported for larger forms i.e. without glass matting etc. and smaller solid-cast forms liable to chip if dropped.

Vaseline should not be used as barrier, and polyesters are affected by contact with moisture/water in the atmosphere

Whereas polyurethane resins are more suitable for delicate castings and there are extra-thin versions to assist intricate pouring, polyester resins tend to have standard viscosities and it can be more difficult to eliminate air bubbles. For example Tiranti’s general purpose polyester resin has a viscosity of 900-1100 mPas, compared to the thinner polyurethane resins at 50-70 mPas.

Working life

Always mark newly-bought resin clearly with the date and make a note of the manufacturer’s recommended shelf life (this is a guide only, see the start of the ‘quick view’ comparisons page for more advice on this). Exposure to air, especially moisture in the air, is something that severely shortens shelf life when it comes to any form of resin. Replacing caps straight after working .. and as often as possible while working .. will help to extend the shelf life way beyond the manufacturer’s guidelines! The containers that resin comes in are not designed for easy pouring of small amounts! In practice one will have to decant a certain amount first into another vessel i.e. a disposable plastic cup (which can be pinched at the side to make a handier pouring point). Whenever possible the decanted resin should be used rather than poured back into the main container.

Pot-life c. 20mins. At 2% catalyst Tiranti’s MP ( multi-purpose ) can be safely demoulded in less than 2 hrs but allow 72hrs-1week for full setting. As an average (this will vary according to type/brand and conditions such as room temperature) there will be 15-20mins working time once mixed; touch-hardening in 25-30mins; demoulding and some mechanical work after a few hours, but full curing can take at least a few days.

Standard addition of catalyst is only 1% by weight but to make portioning easier the catalyst is often supplied in dropper bottles with directions for ‘number of drops per 100ml’ of resin.

When mixing large amounts .. i.e. numbers of litres .. the heat from the exothermic reaction assists proper curing so only 1% catalyst is necessary, whereas when mixing very small amounts .. i.e. under 100ml .. this heat is reduced, so to ensure curing up to 4% of catalyst can be safely added. Adding this amount of catalyst to a large volume of resin would most likely cause cracking due to excessive heat.  ‘Gel coat’ can normally be catalysed adding 2%.

Storing polyester resin A place should be chosen for storing polyester resin which is as cold as possible .. certainly never above 20 degrees C. There may be a problem when storing the resin indoors, particularly if anyone is unusually sensitive to the smell because I’ve noticed that however tightly containers are sealed a trace smell will persist. The only options then are either to wrap the containers within many layers of clingfilm etc. or to store in a secured and sheltered place outside. This can certainly be of benefit during cold weather because there’s no lower temperature limit as far as I know, and the shelf life is improved by cold storage. The resin should be brought back to normal temperature before use because it thickens up when cold.

Additional technical info

If barrier/release is needed (not normally necessary with silicone or vinyl) use polyvinyl alcohol or rape seed oil. There is a special release method for building laminated castings in plaster moulds involving a parting wax and polyvinyl alcohol (see Tiranti website for info).

Polyester resins can be easily coloured with small amounts of standard oil paint without affecting cure. Dry powder pigments can also be used, but the powder should be wetted and thoroughly mixed first with a small amount of uncatalysed resin before mixing it with the main batch. There are commercial polyester colourants available, which are strong, but tend to be expensive.

Resins generally yellow a little after long-term exposure to UV light but a ‘UV stabilizer’ is available for use with polyester resins (added 1% to mix). This used to be available from Tiranti, but not currently (2015).

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.

Clear casting resin is often very pale blue in appearance but gradually loses this while curing. Some clear casting resins can be very ‘air inhibited’ that is any surface exposed to the air during curing will remain tacky .. may not harden completely and needs to be scraped and sanded off once the rest is fully cured.

What it costs and where to get it

Prices will vary considerably according to where you choose to get it. For example if you find versions of polyester resins in craft/hobby shops the price is likely to be unreasonably high! The following is a guide to more responsible prices .. which can be as little as £3.70 per kg for general purpose resin from a specialist supplier such as CFS.

Tiranti prices (2015, inc. VAT) multi-purpose £9.14 per kg, general purpose £9.36 per 1kg, gel coat £11.52 per kg, clear casting AM £13.76 per kg. Buying 5kg amounts will commonly work out 25-30% cheaper. All Tiranti resins (gelcoat, multi-purpose, general-purpose and clear casting) use the same MEKP catalyst (M50) which is supplied in a dropper bottle. For 2% catalyst addition by volume, 100ml of resin would need 50 drops, by weight 100g would need 40 drops. Catalyst sold separately  £2.42 per 50g bottle. Tiranti’s gel coat can be catalysed adding 2% whereas MP, GP and clear should use just 1% for large solid castings (but up to 4% can be used to accelerate small castings). At 2% catalyst MP can be safely demoulded in less than 2 hrs but allow 72hrs-1week for full setting.


CFS prices (2015, inc. VAT) General purpose £6.79 per 1kg inc. catalyst, £20.82 per 5kg inc. catalyst; Easylam (not approved for marine) £18.60 per 5kg inc. catalyst


Specialplasters.co.uk prices (2015, inc. VAT) Clear casting 1kg £9.12 inc. catalyst; SP100 general-purpose ‘laminating resin’ 1kg £8.28, 5kg £22.20 not inc. catalyst

Further info sources

http://www.cfsnet.co.uk The CFS website contains a lot of technical advice and ‘how to’ articles



‘Fibreglass’ is the common name for the strong, lightweight material made by layering pieces of glass-fibre matting generously filled with polyester resin. Another term you might come across a lot is ‘glass-reinforced plastic’ shortened to GRP. The glass matting is designed to soften considerably once the resin infiltrates it (binders are dissolved) so that it will lie properly over contours. The resin needs to be brushed on and worked in with a stiff painting brush.

Chopped strand mat (2015, inc. VAT) standard 300gsm £1.62 sq metre (specialplasters.co.uk) £2.94 sq metre (Tiranti). Matting used in conjunction with MP or GP polyester resin (do not use clear casting resin) for fibreglass lamination.

Just 1 soaked layer of 300gsm matting may be more than strong enough for a small form i.e. up to 20cm; larger than that 2 layers up to 40cm, and larger than that 3 layers. This is a very rough estimation, and it depends of course on whether the object will be load-bearing or not. It will also depend just as much on shape .. concaved or ribbed forms being generally stronger than flat ones.

For a detailed account of how to make a fibreglass cast from a large silicone rubber mould see the later parts of Making a supported silicone mould for a life-size head and casting in fibreglass which you will find in the Methods section under Mouldmaking and casting.

Polyester coating

Because the foam inside Kapa-line foamboard is polyurethane it will accept resins like polyester (whereas styrofoam is dissolved by it). GP (general purpose) polyester resin from Tiranti £9.36 per 1kg, catalyst £2.42 per 50g. Mix in very small (10g) amounts and catalyse at 2% (i.e. 4-5 drops per 10g of resin). Coat using soft brush, careful re. hair loss. Working time is c.14mins in warm room, to 18mins in cooler room.  Average setting time 20mins, ready for light sanding after a few hours. Forms are better left outside to set if conditions are relatively dry. When brushed on foam surface it will partially soak in which means that one coating will produce a reasonably thick, strong shell when the resin is fully cured. Another coating can be added for greater strength but this may start to fill in detail and show brush build-up.

One way of maintaining more control over the smoothness of the coat is to colour the resin so that the build-up can be better seen. This will also assist later painting. I would recommend using dry powder pigment because this will also act partially as a filler and make the cured resin easier to sand. Polyester resin can take up to 10% powder pigment by weight without affecting proper curing.

5 thoughts on “polyester resin

  1. Hi David, what a wonderful resource you have created here. I am looking to pour fill a mould to act as a base for a sculpture. It is pretty big and I wondered what I could throw in there to use less resin. I’m not talking about fillers, but debris (old bits of resin for example) to help fill up the mould. I Wondered what insert materials I could use (plastic, metal, glass?). many thanks.

    • Hello Claire, Sorry about the delay. With polyester resin you need to be especially careful to avoid moisture. If it’s a case of wanting to bulk up really cheaply while adding weight I would normally suggest coarse sand or even gravel .. as long as whatever it is is fully dry! I’m assuming that anything else (i.e. what you’ve listed) would also be ok as long as nothing is chemically active still (greasy, not properly cured etc).

  2. Pingback: Polyester Casting Resin | Purathrive

  3. First I want to say what a wonderful resource you’ve created in this website. Thank you.

    Now my question: If even small proportions of fillers can make resin hard to work with how do kitchen worktop manufacturers produce mixtures with typical proportions of 95% ground-stone and 5% resin?

    • I think this is fairly easy to answer .. firstly the resin used is unlikely to be polyester or PU, rather an industrial epoxy or acrylic. and they probably use both heat-curing, injection and pressure techniques, beyond the means of home users. Secondly the ‘aggregate’ filler is probably very coarse so doesn’t in itself effect the resin.

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