Re: help...? Air Filter ideas needed...nothing fi
White, expanded bead Polystyrene foam (EPS) or styrofoam does dissolve readily with polyester resin. For one-off designs, you can use this foam, but the fiberglassing must be done with epoxy resin, which does not attack the foam. After the part is built, poor gasoline, laquer thinner, acetone or similar solvent into the mold and dissolve away the foam. I've seen many one-off motorcycle gas tanks made this way. It's handy since you don't have to worry about release films or draft angles.
As others said, with polyurethane foam you can use polyester resin since it doesn't attack the foam. You do need release film and draft angles in order to remove the mold without destroying it. This method is best if you want to make more than one since the mold is reusable.