Mod Podge is a sealant that can be used on all sorts of surfaces, including Styrofoam. Before applying it, the surface should first be coated lightly with vegetable oil or mineral oil to help seal any pores in the material so that water doesn't seep into its cell structure and cause it to dry out and crumble. Then apply Mod Podge liberally on top of this when the surface is dry to create a barrier between the shape (or object) you're making and any moisture it may come into contact with. You will need patience because painting on Mod Podge takes time. The dried product should then be polished with some glass paper or sandpaper for a smoother finish.