When working with foam, special care must be taken to use products that will not damage the foam. Many products used in common applications, such as glues and paints, will work on many seemingly delicate materials but eat through even the densest piece of foam. Specially formulated spray paints and glues use materials not harmful to foam. Learn what to look for when working with foam.
What Melts Foam?
A solvent is a liquid able to dissolve another substance. Solvents are used in paints and glues to ensure they stay put after application. Common solvents include acetone (an ingredient in nail polish remover) and methyl acetate (found in glues and paints). All spray paint contains solvents, and many paints contain solvents that will dissolve foam.
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Why Does Foam Dissolve So Easily?
Foam is created when air is used to expand a material such as polystyrene, a plastic. Expanded polystrene foam (commonly called Styrofoam) is used to make decorations, disposable cups and plates, packing materials and other items. Because of the open structure of foam, solvents that otherwise do not dissolve regular polystyrene will dissolve the expanded foam by penetrating the pockets of air and making contact with more of the polystyrene's surface area. For this reason, avoid using paint with the ingredients toluene, xylene and acetone.
What Solvents Are Safe for Foam?
Foam-safe spray paints use a well-known solvent not contained in the list above: water. Water-based paint will adhere to the surface of the foam without dissolving it. To find foam-safe paint, look for the words "water-based" or "H2O" on the front of the can. Krylon produces an alternative to their regular paint called Krylon H2O Latex Spray Paint, and many other brands offer similar products. If you're not sure whether a spray paint is foam-safe, spray a small amount on a piece of scrap foam to check for negative effects. If the paint is harmful, it will begin dissolving the foam after a few minutes.
What if I Can't Find a Water-Based Spray Paint?
When the only spray paint available contains harmful solvents, the surface of the foam can be protected from damage by applying a foam-safe sealer before painting. Apply two coats of polycrylic, a water-based protective finish, to seal the foam. You will then have a surface suitable for painting with any spray paint.
If no other option is available, spray paints containing solvents may work when used alone if applied in a series of very thin coats not penetrating deeply into the foam.