In general terms, Polyester resin contains styrene - and since you are using EPS (Expanded PolyStyrene) foam, the polyester resin acts as a solvent. Acetone and some other organic solvents will do similar things to EPS foam. What you *could* do is see if coating your prototype with an epoxy paint would prevent the Polyester resin from eating the foam (test it on a small piece of foam first, of course). There are other options out there - and it depends on whether you plan to pull molds off of your vehicle, or, do a surface lamination and then somehow strip the internal structure and leave a shell.

Whatever course you decide on, I think it would be well worth your time to consult with a supplier of lamination materials - like the folks at

http://www.fibreglast.com. Just tell them what you're doing and what you want to do, and they should be able to help you choose the right materials for the job.