Styrofoam is a brand of expanded polystyrene foam that's used primarily as packing material but is also common for arts and crafts projects. If you want to decorate Styrofoam with paint, you may face some challenges with the paint adhering to the foam material. Yet there are a handful of paints that can be used to color Styrofoam in a safe and effective way.
Traditional spray paint should never be used on Styrofoam. It quickly deteriorates the foam material much quicker than other paints. However, a number of spray paints are designed to be used with Styrofoam and make for a quick and easy application. Model spray paints made for model toys that are created from acrylic, latex or a combination of both are commonly safe for Styrofoam use. Other craft spray paints advertise that they are safe for Styrofoam use, and are generally water-based. Floral spray paints used for coloring fake floral arrangements are also common.
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Acrylic paint is an effective paint for Styrofoam when applied directly. It's easily applied, although it may take several coats of acrylic paint to achieve your desired color because it will easily seep into the crevices of the foam. Over an extended period, acrylic paint -- like many paints -- can deteriorate the Styrofoam, so avoid it if you're looking for longtime color. Acrylic paint is readily available at many craft stores and comes in numerous colors.
Tempera paint is a water-based craft paint that adheres well to foam objects. Tempera paint is readily available at most art and craft stores. This paint is best used on foam projects that will never have any exposure to water; tempera paint stays water soluble even after it has fully dried. This kind of paint comes in a variety of finishes, giving the painter options for various looks. However, tempera paint is only available in a limited amount of colors. It also doesn't blend well.
Other Painting Options
If you want to ensure that paint evenly colors and covers the delicate surface of Styrofoam, covering your entire Styrofoam piece in a protective coating may be your best option. Covering Styrofoam before painting can make a paint application look smoother, and the paint won't seep into the porous surface and fade in color. Some of the best options for covering Styrofoam are Plaster or Paris or papier mache, or by using a foam sealant before painting. Using a foam sealant, such as Foam Finish, prevents paint from seeping into the foam and creates a smooth surface for you to paint on. Papier mache creates a thick outer coating around the outside of the object. Once either covering has been applied, you can paint your object with whatever paint you want and the Styrofoam will not deteriorate.