Priming
Sign Foam doesn't require priming. Since Sign Foam is already waterproof, it doesn't need primer to seal it or protect it. However, Sign Foam can be primed to provide a smoother surface texture on the finished product. To prime Sign Foam, sand the surface so that it's free from burrs or other imperfections created by the sign-making tools and apply one coat of primer. Allow this first coat to dry thoroughly, then lightly sand it with 320 wet/dry sandpaper. Continue this process until the surface texture meets your requirements.
Standard Painting
Since Sign Foam isn't an organic material and is completely closed-cell, it doesn't have to be protected by paint or other sealants. However, many users may choose to apply paint for the aesthetic appeal of the colored finish. Paint can be applied directly to the surface of the Sign Foam without preparing the surface with a primer or other material. However, Sign Foam̵7;s cell structure will be visible through the paint unless the sign has been primed prior to painting.
Three-Stage Painting
Sign Foam can be painted in a three-stage process similar to the way some automobiles are painted as long as the material being painted is at least 15 pounds in density. For this finishing process, first apply a first coat of laquer primer. Next, apply enamel paint and allow it to thoroughly dry. Finally, apply a coat of automobile-grade urethane. This final coat will dry very hard. Sign Foam that is 15 pounds or more in density doesn't expand sufficiently to cause cracking in this urethane finish.
Painting the Edges
The edges of Sign Foam are porous and can be difficult to paint evenly. Painting the edges evenly may be complicated by using latex paint. Since latex paint solids are suspended in the liquid paint, the solids may clump together when they dry. This clumping creates an inconsistent finish when applied to the porous edges. To avoid this uneven finish, use nonlatex spray paint instead. Since spray paint won't chemically harm the Sign Foam and shouldn't clump together, it will leave a smooth finish on the edges of the Sign Foam.