Finishing Expanded Polystyrene Foam

I was wondering if anyone has any tips on finishing expanded polystyrene (The grainy pebbly stuff))

Unfortunately I need to obtain a smooth surface from this material and I know this is a far from ideal application of this type of foam, but it was the only type of foam that was available to be hot wire CNC’d at my local shop.

In particular any tips on what type of filler I could use to fill in the pitted surface, and what to us to seal it against enamel spray paint?

Thanks in Advance

well I guess I will post my results and become the source on this issue.

yeah, you can use either bondo body filler or wood putty, either will work, sort of. that blue foam wont give you that terrific of a finish to begin with. I understand that its nice and quick to cut it with that wire, but I think youll learn that things can rarely be both nice and quick. Plus cutting it like that is bad for you and the environment.

If you try to spray paint it it will melt so make sure you cover it entirely with body filler, or wood putty, then sand it and coat it with a high build primer and sand it again, then do it over and over until you think its good to go and then sand it again, twice!

Thanks for the reply, but I’m talking about a different type of foam. expanded polystyrene is the same foam that bicycle helmets are made from -it’s puffed-up beads of foam in a tight matrix.Where as the blue stuff is Dow ‘Styrofoam’ which is extruded polystyrene.

I’m sure that since they are both polystyrene they are similarly reactive though.[/i]

What ever you use is going to be a fragile thin shell. Besides bondo try joint compound. It will be easier to sand than bondo. Dilute the first coat with water and brush it on to get some penetration. Let dry. Add a coat, undiluted, sand and repeat till you get the finish you’re after. Keep working up to finer grits of sandpaper with each coat.

hmm that sounds like a solid approach… lots of sanding but solid.

spackle works too- not sure if its quite the same as joint compound, but you can get the stuff that goes on pink and dries white.

Doesn’t Bondo eat polystyrene (but not urethane foams aka yellow foam aka balsa foam)? We are not allowed to use Bondo on campus so I am not familiar with it.

Another option is aquaresin – it will add strength if that is an issue, but could be expensive if you need to cover a large area. It can be dilluted and sparayed, and it is supposed to be non-toxic. You would not need to sand if you use this.

One step that is helpful, but many people neglect, is to poke holes all over the model before coating it with filler, which helps the filler to adhere.

expanded polystyrene (stuff with small beads) is a bitch to work with. in my experience you can never really get a good finish without tons of work. the extruded polystyrene (blu foam) will give a better finish as can be sanded pretty smooth then primed with a latex primer and painted/sanded.

any reason you used the expanded foam?


it was a forced decision. I wanted my modle hot wire cut, and that was the type of foam that the CNC foam cutting service was willing to use.

So far its slow going, just snaded the form in a bit more yesterday and today I will prime, then fill then sand then prime again. It is worse then a bitch to work with.

lessons learned so far…

  1. Never use EPS —EVER!

  2. Foam is a pain in the ass to fill and sand. There is apparently special foam filler --but its difficult to wok and expensive. Regular filler is much harder than the foam… and when you sand it you get raised islands of filler -resulting in a lumpy model. The more you try to fix this the worse you will make it.

  3. Us e the BIN bran primer that comes in the green and white can --the red stuff won’t work it’s shellac based. I found this out after I primed my model with it.

  4. Krylon makes a spray latex paint which apparently fiinishes well… hopefully this will bail my sorry ass out.

  5. avoid hard stuff --work with your foam on top of another piece of foam at all times to prevent it from scratching and gouging — I walked into a door hinge with my model and torn a corner off. Whooops…

Well I hope others can learn from my horrible mistakes, and find some humor in it.

good tips.

indeed, sometimes the right choice upfront in materials, working process, etc. will make or break you down the line. also a good idea to know when to give up and restart. if i were you, i would have restarted with extruded foam as soon as i saw the difficulties in working with EPS.

seems that you’ve learned lots though, which is what school is all about.

just noticed you were from canada. what school are you at?


I would have restarted with extruded foam had time not been an issue. I think cutting and refinishing pink/blue foam would have taken a bit longer, put perhaps would have yielded a better result. I’m not much of a model maker/fabricator so it could have actually turned out worse.

I also noticed you were from Canada and had to check out your site – it looks like you have a pretty sweet gig going. I’m out at Emily Carr. Did you go to school in Canada?

pink/blue foam cuts as easily or easier than expanded stuff. finishes pretty easy to with a quick sand, prime and paint. oh well, you know for next time.

i did go to school in canada, at Carleton University in Ottawa. glad you like my stuff. thanks. how do you find the program at EC? I dont know much about it.


Well, after approximately 30 hours of painting and sanding on a model that is about 24 X14 x 14 and very simplistic on form I’ve decided that EPS pure evil --and hopefully I will never have to work with it again.That said I did learn a ton in a very short time, but I nearly drove myself insane. I would Like to share though that Krylon Latex (branded Krylon H20) is phenomenal. It won’t melt foam, gives superior coverage, and has a very fast flash time. you barely have to leave anytime between coats as long as you don’t load the paint on. If you have to paint any type of foam I highly recommend this paint. It’s only down side is a limited colour pallet.

Back in school, I would use Gesso as a base coat for over the foam. To be safe I’d use a couple coats, then I could do almost anything to it. The Gesso acted like a sealer. At least that was on the Pink and Blue Poly.

I know you said you were using expanded, but someone said maybe they work the same. Any Art store should have a gallon or more for under $20.

yeah… i still use gesso all the time, that stuff is great.