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!



18 thoughts on “Coating styrofoam with polyurethane resin

  1. Hi David, I’ve just joined your blog and you have inspired me to buy foam, jesmonite and a few other goodies with which to experiment. I do 3D art and intend to brush several coats of jesmonite on to foam shapes. I would like to get a smooth finish with a very light grey speckle (similar to kitchen work tops). From what I have gleaned so far, I would mix a light granite aggregate or a metal powder or coloured glass powder into the Jesmonite mix and let it set hard. Obviously I need to experiment to get the final desired result but am I on the right track? Any tips or hints?


  2. Thank you so much for your amazing website!
    I am a student working between the product and fashion design industries…
    This post inspired so much, but I will admit i am a very new resin user!!! So bare with me 🙂
    I am interested in making some sunglasses and shoes out of resin. They do not need to hold up very long, just worn by a thin woman for 30 minutes.
    Would you recommend this type of strategy for low cost and easy fabrication?
    …Molding blue foam into a shoe form, and covering with a polyurethane resin?
    I also do not want a shiny look, would sanding it be an answer to this as I am interested in an opaque look?

    • Goodness .. that’s a hard one, and I don’t have much time. But yes, it stands a chance of working and holding up for 30 mins if there’s no kicking about! I would plan it in terms of making the shoe in two halves i.e. so that the ‘upper’ shape can be more easily hollowed out then stuck on to the ‘sole’ shape, then the whole blocky thing shaped more once you know it fits, if you see what I mean. Gorilla glue to stick the two halves together. Yes, you will definitely need to sand the PU resin as per the examples shown if you want a matte surface. You can colour the resin with powder pigment.

  3. Hi David! Excellent post. So fascinating and inspiring. You are such a great inventor and alchemist, creating new and exiting work methods and combinations of materials! Thank you for sharing this. I can’t wait to see your exhibition. Kind regards, Marika

  4. Great education, as always. I am often inspired by your forms and techniques and am constantly seeing the new methods or shapes fitting into what I’m working on. I am always excited to notice that you have uploaded something new!
    I have another possible matting agent for your polyurethane surface. For something somewhat finer, but still dead matte, you could try the powdered product used for mixing into floor paint in order to give it a textured grit for a non slip floor. It’s fine, white powder texture is beautifully matte but doesn’t take over the surface you are painting. I’m sure you would be able to mix it right into the polyurethane. I’ve used it for years. It comes in a small package & is found in DIY or well stocked, paint stores.

      • Although it’s slightly different packaging, it seems to be the exact thing. The one I use is Behr, as well. I think because it’s grit texture never dissolves, the light is dispersed in all directions. Creating a matte finish. Unlike a gloss surface. But I know you’re aware of that. I have had no problems like that when mixed into a gloss paint. The key is being able to mix enough into your polyurethane to disturb the surface texture. But, you will also have the ability to sprinkle it on just like your sand surface. But here you’ll get a much finer surface and not lose much of your fine textured, original wire brush application. Being much finer than sand, it’s more like a baking soda-like sized grit.

  5. This is a fascinating project…I can’t wait to see more! If you ever go into production with the play set, I’m first in line to buy one. Thanks for sharing your process.

    • Thanks! It’s taking ages! .. and I only really understand what I’m doing some of the time! If it actually ‘works’ with people then yes, producing some form of ‘edition’ could be an option, more so now that I’ve found a way of making components more durable. There will be more here, in due course .. hopefully a lot more.

    • Thanks Marion! By the way, I don’t think I thanked you for writing the great account of the course for your blog a while back! Also .. do you know I’ve got your nice blue cutting mat? I’m keeping it safe .. well, at least I’m not gluing on it! Let me know if you’re over here again and you can have it back. x

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