Gesso

From CosplayWiki

Gesso is a viscous, pasty substance primarily used for priming canvases for painting. It can be used as primer for both acrylic and oil paints. Acrylic gesso can be colored by mixing in acrylic paint with it before applying it to a surface.

Modern gesso is a mix of an acrylic adhesives and calcium carbonate. Small amounts of ammonia or formaldehyde are mixed in as a preservative. Use gesso in a well ventilated area.

It comes in many different sizes, and is useful for tons of things. Gesso is amazingly useful. You can do multiple layers on anything you wish to paint, and sand the gesso down for a smooth finish. It does not melt foam. You will NEED this before you paint foam because many types of paint will just soak into the foam, and possibly melt it.