Rust-Oleum Filler Primer is a hi-build formula that fills in minor imperfections in the surface and improves top coat adhesion and appearance. This product is suitable for use on metal, wood, and fiberglass surfaces. Filler Primer features an advanced spray system that allows you to spray at any angle, even upside down for those hard-to-reach areas. A comfort spray tip with a wider finger pad reduces fatigue caused by continuous spraying.