If you are using epoxy you don't have to put anything on the foam. Just lay your glass and resin right on the foam. If you are using polyester than you have to be real careful. I would take spray adhesive and aluminum foil. Spray the foil and apply to the foam. The foil will have small wrinkels but they will not be seen through the glass cloth. Then wax and put a heavy coat of PVA. Butch