You could try aqua resin instead of fiberglass resin. That might not be so corrosive. Or you could skip the resin all together and go straight to plasti-dip. Might take a few coats (possibly with some sanding in between to take off the high points) depending on how porous the foam is. The best part is, plastidip does come in a variety of colors, including the trademark red, white, and black of the pokeball, so you could build up the layers in the appropriate colors and save yourself some worry over minor scuffs and/or scratches, as the color won't just be a "surface cover" as it would be with traditional paint methods.