A great way to finish foam is to first sand it (use about a 400 paper and wipe it down with a soft damp cloth) and fill in any pits/defects with a lightweight spackling putty and then lightly sand it again with a very fine sandpaper. Then get some silkspan (the stuff used for covering model airplane wings- places like Tower Hobbies sells rolls of it) or very light weight fiberglass cloth and cover the foam with Minwax Polycrylic. Make sure the covering is completely wetted out with the Polycrylic. Once it dries it can be sanded smooth and painted with most all types of paint without worrying about dinging the foam or melting it with the paint.
It's basically a cheap and quick method of low cost fiberglassing without the mess and smell.