Coatings for Rigid Foam, The Basics

The Basics of Coating Rigid Foams

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But please read this article first.

Before you start throwing money around on foam coatings it important to understand some basics.

I will assume you need either protection or texture or both for your project.

FOAM CHEMISTRY

Let's start with the chemistry of foam, you are either going to be coating EPS (Expanded Polystyrene) or polyurethane foam. You should know what you got, but here's a simple test. Polyurethane doesn't dissolve, put any kind of hydrocarbon on it such as alcohol and it remains intact. EPS on the other hand will dissolve and turn into a gooey blob.

Polyurethane is very expensive compared to Styrofoam ® so you don't see it very often. The most common place you see it is florist foam. The green or gray stuff that crumbles by your touch. In my industry you see it on movie sets all the time. That stuff comes in huge blocks called buns and it carved quickly into walls, rocks, status, etc.

On the other hand EPS is ubiquitous. You may drink out of a Styrofoam cup, unpack your computer that has been protected with EPS inserts and then drive on a freeway on ramp that has been formed with massive blocks of EPS called Geo-blocks.

I hope I don't have to say that EPS stands for Expanded which means blown or turned into foam. Polystyrene by itself is simply a common plastic. Often people get lazy and just call it styrene. Those model kits of cars you got when you were a kid are styrene.

COATING CHEMISTRY

When it comes to coatings you have several choices, but the basic ones are:

PLASTER: Plasters and gypsums come in a variety of formulas. Maybe all your project needs is a coat of industrial plaster like you would find on a stucco house. You can probably find what you need at your local hardware store. Just fine for Polyurethane or EPS.

EPOXY: Either as a coating or laminatiolamanitation. Comes in two parts which you mix together. Very expensive but great for smooth surfaces. To test this theory on your project go to the hardware store and buy a epoxy glue kit for five bucks. The fumes are toxic so if you buy a big kit please make sure you have plenty of fresh air. Works great on polyurethane and should work well on EPS. Epoxy rarely melts Styrofoam ®.

SIDE STORY: Many years ago I was opening a show in Texas. I spent the entire morning finding a five gallon epoxy kit so the set guys could make some last minute repairs. It cost $600. One guy wasn't thinking and mixed the entire kit somehow believing it would last for the entire day. Twenty minutes later it was a solid mass with the mixer permanently embedded in it. I doubt he used more than a pint of it.

POLYURETHANE: OK, it is always best to use the same chemistry with the foam you are trying to coat. This is a fancy way of saying polyurethane is a good coating for polyurethane foam. Well, it is and it isn't simply because rigid polyurethane is difficult to control and “sculpt”. I don't think anyone does this in their shop, but it is still a good theory. Most of the time it will eat EPS.

POLYESTER: This is normal fiberglass resin. Works great on polyurethane foam. In fact on building facades and props in amusement parks it it a common practice to simply coat a big chunk of polyurethane foam with fiberglass and never separate the two. I highly recommend this approach if you have idiot humans and maybe animals coming in contact with your project and you need a lot of strength. It eats EPS.

ACRYLIC: You just don't see this very often so that's why I developed it as a coating for EPS. Steve' foam coat is simply a bucket of acrylic plastic in liquid form with a filler to give it texture and strength. It does not eat EPS so some of my customers use it to seal their EPS before applying a layer of fiberglass over the top of it.

The advantage of Steve's Foam Coat is it doesn't have the harsh or dangerous fumes like you have with epoxy or polyester. It does have some body so it is not recommended for smooth surfaces. But it can be sanded, but it's like sanding a hardwood like maple.

NO UNIVERSALS

To my limited knowledge there is no universal product that can be applied in all situations. A good shop will have knowledge in all materials and a small supply of each so they can take care of their customers. I'd say to buy a quart of Steve' Foam Coat so you can play with it and know how it works before you order more.

Steve Biggs

5-13-2024