Coating styrofoam with polyurethane resin

These are the latest forms I’ve been making for my .. not-quite-working-title .. Ridiculously Organic Construction Toy. For this I’ve been creating simulations of eroded rock and driftwood cast in resin, twisted Pvc branches covered in fake moss and lichen, corals, leaf clusters and strands of seaweed made from latex etc. But I also wanted to include some play elements which are more obviously scaled down, such as these brickwork ruin pieces. The best way of picturing the whole idea is to think of aquarium or reptile tank accessories and then imagine getting a large collection of these instead of a box of Lego. I’m still working on the question of how exactly the ‘construction’ is achieved .. i.e. how such components will be fixed together when playing .. but as part of the system I’m working on an artificial ‘mud’ which I’m hoping will solve part of it.

ruin fragments in resin-coated styrofoam

The forms above were cut/carved in regular blue styrofoam, textured using a heavy-duty wire brush and then coated in polyurethane resin. There’s a bit more to the ‘painting’ process .. something new I haven’t tried before .. but I’ll come to that. If properly done the method of resin coating makes the forms unbelievably strong! .. perhaps not enough to survive little children, but certainly any adult wear-and-tear.

Making a brickwork arch in styrofoam

These two photos illustrate other forms intended for the collection and the process of making them. I’ve described this method of form-making in more detail in Shaping styrofoam. The arch piece above started with a Pvc template, which I used to help sand a block shape. I found I had to make a separate drawing template (the one at the bottom) just in order to inscribe the brick pattern onto the styrofoam shape. Then I used the special diamond needle files pictured to scratch out the brickwork divisions at the right thickness. I wanted these pieces to be 1:12, i.e. usual dollshouse scale, but I’ll eventually use a mixture of scales.

Making a brickwork niche in styrofoam

To make the ruined ‘niche’ shapes above I also used the method I described in Shaping styrofoam of using a curved sander to create the concaves. I roughed out very deep channels for the mortar lines, because these will become partially filled with coloured resin .. and this is what gives the pieces unusual strength. I found it was better to make all the channels before attacking with the wire brush, because I made the pitted texture mainly by hitting or pressing with the brush. This peppers the foam with deep holes and it may fragment a bit too much if the channels are made afterwards.

diamond needle files

Here is a close-up of the type of file I’ve found to work best for detailing foam. These have a ‘diamond coated’ surface which has more of an effect on relatively soft materials than the other, cheaper, form of needle file which is just ‘toothed’, grooved metal.

wire brushes useful for texturing rigid foam

I usually use the smaller brushes pictured above when working with the more delicate polyurethane foam in Kapa-line foamboard, but styrofoam has a tougher surface .. the heavier wire brush has more effect. Importantly, the action in this case is not a brushing or sweeping one, it’s more hitting downwards and rocking around .. I call it ‘scumbling’.

styrofoam 'ruin' fragments

Now to get to the main point of this article! Of the polyurethane resins I most often use (Sika’s Biresin G26 and Tomps’ Fast Cast) I know that both can be used in the following way, but Tomps Fast Cast is best because it’s a little thinner, powder pigment mixes better into it, and according to Tomps it is designed to cure properly in very small amounts or in very thin layers. This is not the case with all polyurethane resins. I’m basically making a very quick-setting paint with it, and because it’s quick-setting it has to be done a little at a time. To dose both resin parts I use disposable plastic pipettes (which are available from a few places online) and usually work with not more than 2ml of each part at a time. I can normally manage to use up to 4ml before it thickens too much. Because there’s usually no time spare to clean the palette surface before it sets I use a ceramic tile which can be scraped clean afterwards. There’s always just enough time to clean the brush though, and this can be quickly done with acetone.

Coating styrofoam with polyurethane resin and pigment

Here I’ve dosed 1ml of each resin part together on the tile, added a small amount of powder pigment, mixed the whole together with a synthetic-hair paintbrush and used the same brush to paint the foam. Synthetic is best because the hairs will be rigid enough to push the pigmented resin into deep pattern, but full and fine enough to hold a lot of the paint. Powder pigment is the best form of colour to use .. strong colour, inexpensive, available .. and I usually find that it mixes better into resin than it does with water!

The polyurethane resin has no effect on styrofoam (unlike polyester resin), it will cure hard and ‘fused’ to the surface, and it’s done .. that is, it’s touch-hard and ready for further work .. in about 15 minutes! Whereas regular paint such as acrylic will infiltrate more and contract as it dries, polyurethane resin does less of both so there will be a little ‘smoothing over’ of fine surface detail. It will also be a gloss finish! .. which I don’t like, would never choose, and at the moment I’m experimenting with the different  ways of dealing with this. There is no matting additive for polyurethane resin, and regardless of which pigment or filler is mixed with it, the top surface exposed to the air will always be glossy. Obviously painting over with another matte paint, such as a good acrylic, is an option .. but polyurethane needs a lot of preparation if the paint coat is to resist a lot of handling and this is made difficult by such a patterned/textured surface.

One possible solution is to use my own version of cold powder coating. If you google ‘powder coating’ you will find that this refers to an industrial painting process in which fine thermoplastic powder is melted onto metal to create a durable surface. It’s very like the enamelling that you might have done at school, with coloured glass powder on a copper plate, melted in a small oven. My version does not require heat, and it’s perhaps more related to the model-making practice of scattering granules into glue to create a surface .. but it does share some of the surprising durability of these other methods!

crushed brick

Below is a close-up of the styrofoam ‘ruin’ forms after coating. I first gave the bare styrofoam an undercoating of resin mixed with black pigment, and then a second coat without any pigment, covering a small area at a time. While each portion was still wet I sprinkled a mix of finely crushed brick and sand onto the resin. I’m fortunate in that, living close to the Thames beach, I can pick up fragments of any colour of brick, illustrated above. Since these have already been broken down by the elements they are much easier to crush to a powder using mortar and pestle.

detail of brickwork surface done with 'powder coating' method

While working I could see that the particles were readily sinking into the thin coating of resin, and when the excess is shaken off after a few minutes the powdery top layer still adheres strongly. Polyurethane resin is a strong adhesive, especially if the dust or particles are porous and jagged. Having tested the strength of the surface once the resin cured I have little doubt that it is permanent. I still have to do some paint finishing on these pieces, emphasizing contrasts and colours and giving more ‘speckle’, but I have no worries about regular acrylic paint attaching itself on top. The greatest bonus in this particular case is that these pieces have a lot of the look and feel of real brick .. because that’s what it is!


More on gluing, surfacing and repairing styrofoam

I’ve been slowly adding to my page on Shaping styrofoam in the Materials section, partly because I saw that it was one of the most visited pages. I wanted to consolidate the scraps of knowledge I’ve gained over the years on gluing styrofoam and the range of options for finishing or reinforcing the surface. I also wanted to look into a more satisfactory method of making repairs which I found, as you’ll see from the photos  .. well, very satisfying!

More on gluing styrofoam

I’ve bonded pieces of styrofoam together with a strong, double-sided carpet tape for years and they’re actually much more permanent than I’d first imagined. I have composite forms made many years ago which have been well-used as teaching examples and show no signs of coming apart! A few observations are necessary though! .. the two surfaces have to be smooth and flat against each other; if these surfaces have been sanded to make them fit, all dust should be removed, ideally vacuumed using a brush attachment. For the same reason the best bond occurs between the slightly ‘skinned’ surfaces of the sheet as it comes. When using the tape it’s particularly important to avoid placing it too near to where the styrofoam will be carved or sanded if one wants a seamless join .. but this applies almost equally when using most glues. But if it’s impossible to predict, or avoid, gluing in an area which will later be carved through or sanded, I’ve found some of the following options most suitable because they offer the least resistance.

For example what works surprisingly well as a bond between flat styrofoam sheets is spraymount, by which I mean the permanent spray-glue types from 3M. 3M’s ‘Craft Mount’, which is the strongest of their range, is particularly suited because it has a little more body. It should be sprayed on both sides to be joined (different to how it’s normally used), and it’s best to wait a little i.e. half a minute, before the two pieces are firmly pressed together. As with all contact glues, even the ‘instant’ ones, the bond will hold straight away but needs a day or two to get stronger. But the same applies with this as with double-sided tape .. the surfaces must be completely flat against each other and dust-free to bond strongly enough! Spraymount never sets completely hard, even though it’s grip is strong. When cutting through a glued edge the glue may catch the blade a bit but one can get used to this. It will sand reasonably well, especially if the tackiness gets mixed with styrofoam dust. For the same reason, I’ve found that it can help to dust the piece with talc while sanding.

If you want to glue pieces of styrofoam together which do not lie completely flat against each other .. you will need something ‘gap-filling’. There’s hardly anything more gap-filling and instant that hot-melt glue! Contrary to what you might have read elsewhere, hot-melt glue will work reasonably well with styrofoam .. as long as it’s not too hot! One just has to know how to manipulate it. Polystyrene foam is much more heat-sensitive than polyurethane foam .. it softens already at 90°C (193°F), while the temperature of hot-melt glue is around 180°C or more. So there is bound to be some melting of the styrofoam surface, but if you experiment and are careful you can find just the right number of seconds to wait before pressing the surfaces together, cooling the glue down a little but not so much that it fails to stick. Obviously it’s not possible to cover a whole surface, even a small one, with hot glue before most of it hardens. Think of this more like ‘riveting’ .. applying very quick spots of hot glue which will secure the pieces at those points once the glue has cooled. This method is definitely not for carving into or sanding though.

Another option is provided by the ever multi-tasking polyurethane! It’s common practice in scenic work and prop-building to use a rigid-setting, 2-part polyurethane foam to bond other foams such as styrofoam together. A small amount of the two parts must be mixed together in the directed ratio and then brushed quickly onto the joint before the foam pieces are pressed together. The resulting foam will expand to fill any gaps in the joint but the pieces must be held firmly in position while this is happening, otherwise they’ll be forced apart. Often this is easier said than done because the expanding foam exerts quite a force, but generally with these polyurethane foams and glues the less gap there is the stronger the joint will be. Prop-makers and sculptors are more likely to use the foams which are supplied in two separate parts and which need to be manually mixed .. a good source is a sculptor’s supplier such as Tiranti .. but the ‘instant foam’ tube cartridges sold for wall repairs or insulation are similar.

Gorilla glue is yet another option .. a polyurethane glue that comes as one part, so no mixing is necessary, and which foams while curing on contact with moisture. What this means is that both surfaces to be glued need a light misting first .. best with a small pump spray. The glue needs to be applied thinly to just one of the surfaces before the two pieces are pressed together. Below are two blocks of styrofoam already sprayed .. just enough to dampen the surface, certainly not dripping wet!

Gorilla Glue test, styrofoam moistened

Gorilla Glue can be deposited in spots if preferred, and these will spread once the pieces are squeezed together. It is not necessary to spread the glue beforehand over the whole surface but here I used a coffee stirrer just to distribute it better. What is very important to note (and not included in the manufacturer’s directions!) is that the glue should be dropped from the nozzle without letting it touch the wet surface! If it does moisture will contaminate the bottle and some of the glue will set inside .. as I found out when I tested it for the first time!

Gorilla Glue test, applying the glue

Below, I have weighted down the two pieces with a solid metal block. This was sufficient, although where possible it is always better to clamp the pieces together (sandwiched between flat pieces of wood to protect the styrofoam). The expansion occurs after a minute or so, 3-4 times in volume according to the manufacturer, and if the pieces are properly clamped the excess is forced outwards rather than upwards. If, because of the shape of the pieces, it is difficult to clamp without damaging the surface, masking tape or cling-film could be used to bind them together but these could give way a little and there may well some shifting as the foam expands. I had to correct the alignment of these blocks a few times before the foam stopped moving them.

Gorilla Glue test, weighting down

A couple of hours are needed for the glue to cure. Here below is a section through the seam line, cut with a knife then sanded. If it is properly cured the glue sands through well …. although it is different to the styrofoam it is much more alike than other glues. Also the bond is as strong as they say it is! I think polyurethane grabs on the styrofoam particularly well .. at least, I couldn’t pull the blocks apart.

Gorilla Glue test, section through the seam

More on surface treatments

If the surface is going to be reasonably protected .. I mean, if it’s not going to be handled and if it’s shielded from knocks etc. .. a good coat of acrylic paint may suffice. Acrylic paint will toughen the surface slightly, but by no means protect it from any handling, even if careful. Coating first with non-waterproof Pva glue and then painting will do a little more, and the same applies if coated with Pva or a water-based varnish afterwards. But the surface will still be very susceptible to scratches or dents.

If Paverpol is added to the paint mix .. more than half by volume .. greater surface strength can be achieved. Paverpol is a Pva-type medium manufactured in the US mainly for the hobby market and is intended for painting on fabrics or foams to give them a much tougher surface. For example fabric will become rigid and almost ‘resin-hard’ if soaked with it and the manufacturer recommends it for outdoor sculpture. It will not impart the same degree of toughness when painted on styrofoam because it doesn’t infiltrate far into the surface, but a couple of coats using Paverpol will be stronger than either regular Pva or acrylic alone. On its own Paverpol is quite viscous (a little more so than straight Pva glue usually is) but with careful brushing can be worked into a detailed surface without clogging it noticeably. In fact, whether applied straight or mixed with either powder pigment or acrylic paint for an opaque colour, the first coat will emphasise much of the porous foam surface and appear matt with an even roughness. If this is allowed to dry completely (needing a good few hours or preferably a whole day) and then painted again, the pores will start to be filled. Especially if combined with careful sanding between each coat using a medium grit sandpaper (for example 120-200 grit) a fairly smooth finish can be achieved. This must be applied as evenly as possible because raised streaks in the Paverpol will be tough to sand down!

Paverpol fabric strengthening medium

The next option is either painting or coating with a ‘polyfilla’. I make this distinction between ‘painting or coating’ because certain polyfillas can be thinned with water to a gesso-like consistency without losing too much of their strength and can be smoothly painted on. Polycell Fine Surface Polyfilla is in my experience the best of these. It is bought ready-made in tubs; it’s particularly smooth; thins evenly with water; sticks very well to sanded styrofoam and is far less subject to shrinking and cracking than other types. It also retains a little flexibility when dry. To get an even, lump-free cream with water it’s best to ‘condition’ some of it first either in a bowl with a spoon or on a clean glass plate with a spatula or palette knife. As you work it around it will become more liquid and if this is continued while adding water a few drops at a time you will gradually get something like the consistency of gesso, without any lumps.

The forms below were carved/sanded in styrofoam; the sanding stage finished with a flexible foam-backed sanding pad (see below) to achieve the smoothest surface. They were then painted with a thin coat of Polycell’s Fine Surface and left to dry for a day. After this a second coat was applied and then a third, which was left as before. The final treatment before painting was a careful sanding with a very coarse (60 grit) sandpaper which imparted a visible grain, softened with an overall rub using finer (240-300 grit) sanding cloth. The surface achieved with this polyfilla is not any tougher than the Paverpol, but it is much easier to sand, making it more ideal if the aim is an even, glass-smooth one. I painted these with Humbrol matt enamel, which is thin enough not to clog surface detail or add any additional texture but opaque enough for a rich and streak-free coverage.

Finished forms in styrofoam, coated in polyfilla, re-sanded and painted

I waited a few days for the paint to reach maximum hardness (although thin, the enamel dries very hard and durable) then rubbed carefully with a fine sanding sponge (these look similar to kitchen washing pads) to bring out some of the grain.

But an equally smooth and even harder surface can be achieved using a more specialized material. Casting resin is not usually considered as a medium for thin coating, mainly because it’s thought that it will fail to cure properly under a certain thickness. This may be true of some, but I’ve used both general-purpose (GP) polyester resin and special types of polyurethane resin many times for this, mixing in very small amounts and applied like a varnish .. with consistently good results! Polyester resin cannot be used to coat styrofoam because the styrene part of it will eat into the surface. However, polyester resin will work perfectly on polyurethane foam and, with predictable logic, polyurethane resin works perfectly on polystyrene foams such as styrofoam!

Uncoated and coated styrofoam 'heads'

The ‘head’ on the right above was painted with a small amount of Tomps Fast Cast polyurethane resin, first mixed together quickly and then worked with a soft brush into the styrofoam surface. I chose this brand because of its low viscosity, designed to cure even in very thin sections. After about an hour this cured enough to be sanded, just to even out the surface a little, before being coated again. More careful sanding followed, and finally one more coat. The overall coating is now akin to eggshell in thickness and hardness .. but in important respects stronger because it is supported uniformly by the rigid foam underneath. An ‘industrial’ ceramic-look smoothness can be achieved using a mixture of careful rubbing with special sanding ‘cloth’ (blue/green below) or thin layers cut from the surface of a nail parlour sanding block. These will flex with the shape, unlike sandpaper. Polyurethane resin sands surprisingly well for something so hard .. even easier to sand than some fillers!

Foam-backed and cloth-backed sanding materials

Below I’ve included the stages in the shaping of these ‘heads’ .. starting with a block shaped on a template, then sanding away from a centre line.

Stages in styrofoam shaping

The concaves for the eyes were started with a curved sanding tool, similar to the one used on the form at the beginning of Shaping styrofoam. These are easy to make, involving only a thin strip of sandpaper glued to a former.

Custom sanding tools for making concave shapes in foam

Major repairs or alterations to styrofoam forms

It is a long-established practice amongst carpenters and, in more recent times, restorers to neatly excise a damaged area .. i.e. to cut it neatly out .., glue a new block of the same material in its place, and then reshape this rather than ‘bodging’ with something else like a filler. Most fillers tend to shrink, to varying extent .. inevitable if they rely for hardening on the evaporation of a solvent. They usually can’t be laid on too thickly otherwise they can take an age to harden, and it may take a few goes to fill a deep repair properly. The other disadvantage of using fillers, especially to repair or alter styrofoam is that when set the filler material reacts very differently to sanding than the surrounding foam. It is usually much harder and inflexible, making a seamless transition difficult. The much better way to do it .. patching in with the same material .. is illustrated below.

This test form was made in the same way as one of the components described above .. first rough sanded and then finished with a 120 grit sanding block.

styrofoam repair demo test form

Below is some stylised ‘damage’ ..

styrofoam repair demo 'damaged' form

I sanded away a broad channel, making sure that the bottom of it was flat and smooth. The side edges are not parallel but a little tapered, so that a wedge-shaped block can be slid into position. This means that the block doesn’t need to be perfectly accurate in width.

styrofoam repair demo, damage cut out

In this case I secured the block with double-sided carpet tape. I’ve found Ultratape ‘Rhino’ very reliable so far. The block needs to be slid into position, as I’ve said, but not pressed down until it’s tight against the channel edges. I also slanted the edges of the block a little i.e. making the top surface a little bigger than the bottom, so that when pressed down it squeezes even more tightly into place. Styrofoam will compress a little to make this possible.

styrofoam repair demo 'patch' inserted

Then it was just a matter of re-attaching the semi-circular templates either end of the form (see earlier) to protect the undamaged surface and sanding the block down to the same point.

styrofoam repair demo 'sanded flush'

Just out of interest I tried similar repairs using Gorilla Glue as a filler ..thinking that, since the glue takes so well to the styrofoam and cures with a similar composition it might be an ideal solution. Unfortunately not!

Gorilla glue as filler, test piece

I made some notches with the scalpel, spray-moistened the styrofoam surface, then laid the glue into them making sure to spread it completely over the edges of the cuts. I also spray-moistened the topside of the glue after this was done.

Gorilla Glue as filler, cured foam

I waited a little more than two hours, during which the glue had expanded .. as the manufacturer says .. 3-4 times in volume. The first few millimetres of the cured foam was fine-pored and sandable, ideal in fact, but deeper down the bubbles were much bigger and the consistency soft and fibrous. It was like trying to sand bread! The foam had become weaker and more irregular because it was allowed to expand too much and I’m guessing that it would work better as a sandable filler if it were more confined i.e. by wrapping a non-stick covering over the form before the expansion starts.

Gorilla Glue as filler, sanded down