Finishing EPS

Hi Guys I’m looking to build a hollow egg shaped cocoon for a project I am currently working on. Its about 1000mm long by 600mm wide. I have narrowed the core material down to EPS foam which will be cut and shaped (although I am still open to other material suggestions)

I was wondering what you guys could recommend as a hard durable finish to cover the egg with, and what the easiest way to achieve this would be? Would it be possible to just cover it in resin and sand it down to a smooth finish and spray it? I have seen others spraying with polyurethane elastomer , what exactly is this?

Any Ideas will be gratefully received


EPS foam is not the easiest to work with. If your goal is a smooth outside finish, expect to spend a lot of time prepping for paint.
Whether EPS, blue or yellow foam you will likely have to coat in a drywall, spackle, plaster based coating to fill voids, sand to smooth, and paint.

Here is a “How to Finish YELLOW FOAM” thread:

Have you considered making a mold and thermoforming? If symmetric and you have access to a vacuum form machine big enough, it could save you a lot of sanding. If you are working on interior and exterior together, foam may be your best choice.

expect to spend a lot of time

No matter how you look at it, you are going to be doing a quite bit of sand and fill work. Always remember that your LABOR is going to be more expensive than your materials.

EPS is certainly cheap, and a pain in the arse to work with, but if you are only going to build ONE part it is one way to go. To save all the hassle of spackling and sanding, you might consider using EPOXY resin and fiberglass CLOTH (not mat). Epoxies will not attack the EPS, saving you the time, and materials required to protect it. A hidden advantage to the epoxy/EPS combination is that when you are ready remove the foam plug, all you have to do is pour a cup of acetone into the cavity and it will dissolve the foam.

I’m guessing you are going to be using sheet material to develop your “plug”. I would cut out circular cross sections with a hole at the center, and then mount the sections on a section of pipe (shish kabob style) using a pressure sensitive tape to adhere the sections each other as you go. Most spray adhesives will attack the EPS. When you have assembled this mess mount it in a crude fixture that will allow you to rotate it (as in lathe), and using a template knock the corners off of the sections to reduce it to a smooth(er) surface. Starting with 40 grit sand paper start to develop the surface, finish up with 80 grit (no reason at all to go any smoother). Use your template to control the sanding process.

Using polyurethane sheet foam material, while more expensive, would allow you to use cheap(er), and more readily available, polyester resin; polyurethane foam is resistant to polyester. In addition, your plug sections could be adhered with spray adhesives.

You will need to determine how thick the shell is to be and allow for that in the foam plug. i.e. if the cross-sectional diameter to the shell is to be 600mm, and the material thickness is to be 3mm, reduce the diameter to 594mm (allowing 3mm on both “sides” of the diameter).

Cut the cloth on the “bias”. i.e. roll out the cloth on a table and cut it at a 45 degree angle. When cut in this manner fiberglass cloth will readily conform to compound surfaces (and I would consider an “egg” shape to be compound).

In addition to the advantage of cloth conforming to the surface is that cloth can be laid out on the form DRY (when it is far easier to handle) and subsequently do the “wet out” in place.

With this method, it would be possible to do one half of your egg with one piece of cloth, allow it to cure, and then do the other side.

But it is going to take more than one “lamination” to get any kind of usable materiel thickness, unless you are only going for a visual part; as in no structural load. Typically a single layer of 1-1/2 ounce cloth, laminated with resin, will yield a thickness of roughly .015-.020" when rolled out.

I’m not sure what spray on PU elastomers you observed being sprayed on … you should ask them what it is.

With regard to any “sprayed on” material it will not be inherently strong by itself; think egg shell; easy to crack. To get the strength you will need in an object of this size you are gong to need a “reinforcement”(fiberglass cloth). A vacuum formed thermoplastic ABS shell would be strong enough, but the mold would still require a “pattern” of the shape be made to subsequently make a mold from, you would require two halves that would have to be bonded together and sanded to shape. If you opted to vacuum form over a male mold, the “second surface” would still require considerable sanding to achieve final form in addition to the joining and finishing process.

Wow Lmo & 51, thanks for putting so much time and effort into a great answer! The fiberglass cloth is currently leading at the moment, but, for some reason I think it may be overkill. We Basically want a hard shell over and inside the EPS “egg” which will have about a 3" wall thickness at its thinnest point. The “egg” will have a hole in both sides for access. We don’t need it to be super strong as it will be protected by a wooden frame, just strong enough to take the odd knock. Have you any knowledge or experience of just using epoxy without the glass? We don’t need a super smooth finish, in fact a slightly uneven textured surface may add to the piece.


This may be stating the obvious, but have you considered carving, or machining out of wood, even MDF if you are building form with glued layers? You can spend more time resolving the form expectation, and less time on final appearance/finishing. Sanding sealer, paint and you’re there once you have completed your desired form. Even epoxy is not easy to finish. Certainly a non-smooth finish saves on sanding and your lungs!

Yeah wood would be way too heavy, time consuming and expensive. We want to make a few of these and want to do it at a low cost and in the simplest way possible!
But thanks for the incredibly quick answer!

We Basically want a hard shell over > and inside > the EPS “egg” which will have about a 3" wall thickness at its thinnest point.

We want to make a few of these and want to do it at a low cost and in the simplest way possible!

Well, that certainly changed the game.

A 1000 x 600mm egg with a 3" MINIMUM wall thickness?

How many is a few? If more than one, it’s mold making time.

Hi Lmo
It’s actually not as much surface area as it sounds as you can see from the attached image. Still a lot but achievable I think.

hey freddie,

Sorry for the delay getting back to you. The shape you have here is simple to mold, but by your finish specification, complex.

One note: if this were a “production” part, molded using EPS foam (or anything else for that matter), you would definitely need to eliminate any parallel surfaces (interior and exterior); parallel surfaces would hang up in the mold making the part hard to release. Add 1 degree of draft.

The problem:

  1. A molded EPS foam component with a “hard” exterior is going to be costly; EPS will not tolerate (chemically) too many coatings, without costly pre-prep to make it resistant. Even then, a simple spray-on coating will not provide much resistance to impact, and you would have only minimal control of the surface thickness and finish; at this point any sanding operations would become prohibitively expensive.

  2. Aside from the expenses of molding an interior and exterior shell (one mold each), bonding them together, and then filling with EPS, any “shell” you form (thermoSET, or themoPLASTIC) would require tooling to trim and hold the parts, and would not likely tolerate the temperatures required by EPS molding process without deformation (increasing the likelihood of quality control issues).

Without giving away any proprietary information, can you say what the use of the part is? Is EPS suitable without the “shell”?

Considering the expense of the added processes required to put a skin on EPS, would polyurethane foam be a suitable replacement?

Consider; two-piece molded shell (jigged and bonded together) and back filled with liquid PU foam (at room temperature).