Polystyrene sculpting

Hello everybody! At first, let me introduce myself (and don't read if you don't give a damn about my life).

So I started to make the big rear shell of the portal gun, made of polystyrene foam ("polystyrène expanse" in french, is it the right translation?), this is my progress.



When finished, I'll cover it with a coating, like building coating (like that), then sand down and finally paint. But I've a problem. This shell, as its name suggests, has to be hollow. So I must dig into the polystyrene, to make it hollow. And I'm afraid to break the shell using the tools above. So:

• What tool do I need to minimize the risks of breakage?
• Somebody suggested to coat the shell before digging. Good idea or not?

I hope you'll help me, and I think I'll open a thread about my replica on this forum, once this problem is solved. :3


Thank you so much for your help,


dolf.

I'd say you'd want to coat it in bondo (or other automotive body filler), and then hollow out the inside. Depending on the type of material you are putting the filler on, it has a chance to dissolve the base material, so I'd recommend testing it out on a scrap piece first. If it does dissolve it, then you'll need to seal it first then put on the bondo. Once its dry sand it smooth and you should be able to hollow out the inside. I hope this was helpful.

So, if I understood (I'm french x) ), I have first to coat it with synto (a polyester mastic, a french body filler, bondo doesn't exist in france), and if the synto can dissolve my polystyrene, firstly seal it (I didn't understand the meaning of this word, sorry :° ), then coat it, and then hollow out? Sorry If i didn't understand everything, some subtleties of your own language "beyond me" a little. :3

I am in no way an expert on coating polystyrene foam, but I am planning such a project so I read up on it, experimented some, and this is what I found:

Polyester - based fillers or urethane resin could dissolve the polystyrene.

I think that the most durable coating for foam would be epoxy resin + some type or filler powders. Go to a store that sells equipment for boat building and repair and get some "Epoxy primer", or just epoxy. The "primer" is tinted and just a little bit thicker. Paint a couple of layers. Perhaps you would have to fill it with something to make it thicker: Microballoons, microfibers, talcum powder and/or cabosil (colloidal silica), which should also be available in the boat building store.
There is a "101 Mini Pack" from West System, with both epoxy, fillers. mixing syringes, mixing cups, mixing sticks and protective gloves.

Look also at the foam-coating thread in the Costumes forum. That is for coating flexible foam, so it is not the exact type. They use wood glue as primer coated with urethane resin, but I don't believe that it could be as adhesive or durable as epoxy. They recommend also that you use a type of urethane that is made for rotocasting, but I don't think that is easy to find in France. (I have tried this method with ordinary casting resin from Esprit Composites (French brand), but I made a complete mess... )

Another recipe for coating that is often used for polystyrene foam is called "scenic dope". It is based on latex paint and latex caulk + optional filler. It is more often used for larger set pieces and not props that should be handled. There are several recipes with different properties. There are a few threads about it on the forum and elsewhere on the web.
This is much cheaper and much less toxic than the other options. If you are a beginner and with limited funds, then this might be the best option.

Not 100% sure if this would work, but take your foam shell, cover it is bondo as suggested and then fiber glass that. Once set, turn it over and use acetone to eat the foam out. Result: One hollow shell with a perfect shaped interior and no risk of breaking through during hollowing.

Polyester resin, the stuff commonly used for fiberglass applications can dissolve polystyrene foam. Body fillers such as Bondo can also dissolve the foam as well. Urethane resin does not dissolve polystyrene foam and can be brushed on in coats to make a hard shell. I personally had a few problem areas on my Portal Gun shells where the urethane resin didn't bond to the foam very well, perhaps better prep work may fix that. I may have had my foam surface sanded too smooth for it to bond to it well. I have seen other people seal the foam with simple acrylic paint which seals the foam and allows you to use body filler or other resins without harm. To hollow out my shells, I used a rotary tool with a course sanding drum, it will however make a huge mess! Also, don't forget when using resins and sanding to be wearing a respirator !

Thank you for your answers!

weaselhammer said:

Urethane resin does not dissolve polystyrene foam

Click to expand...

That depends on the resin that you use.

The french brand of urethane resin, "Esprit Composites" that I tried, did dissolve the foam somewhat. It did not eat through it as fast as with acetone or "poly cement", but it did leave a small gap between the resin and the foam.
It did also eat through my polystyrene cups (not foam) that I used for pouring. The cups held together for one pull (small mold), but not for two.

As someone who's midway through a Portal Gun build, I'd highly recommend cutting the inside out first and then shaping the outside. Doing it this way means that you'll get a snug fit to the barrel and actually an accurate shape on the outside too. Obviously you probably don't want to start again (it is frustrating!) so instead I'd say you should soak the foam in a 1:10 mix of PVA glue and water, then a coat of 1:5 PVA and water, and then start using some kind of "smooth-finish" filler mixed with a little PVA for strength.
Hope this helps!