Epoxy resins won't dissolve styrene foams; I usually cover my patterns with one layer of fiberglass cloth and whatever epoxy resin I have handy. I generally have some cans of epoxy laying around that are out of date, or had their seals broken a year or more ago, or are otherwise questionable for structural use, and one layer of cloth doesn't call for much resin, but epoxies cost roughly double what polyester resins cost so you have to weigh the cheaper foam against the pricier resin if you're buying materials rather than using leftovers. To finish after cure, Featherfill or the like will fill the cloth pattern nicely, and from there, sand 'till you're happy.
One layer of light cloth gives you a firm surface to work against, but it thickens the surface by about .032" (which by amazing math coincidence is also about 1/32") but I don't often compensate by making my pattern smaller because the mold will shrink a tiny bit and the part will shrink another tiny bit, so the pattern should be a trifle bigger than the finished part anyway.
horizenjob wrote:
I was going to suggest that large plastic shrink wrap...
...which is available in big sheets at every hardware store in Autumn/Winter (and many hardware stores year 'round) as a cover for drafty windows. But be warned, the activation temp for the shrink wrap isn't far off from the goober temp of the styrene foam, so go easy with that hair drier.