To paint a fiberglass truck cap, begin by cleaning grease, dirt and residue from the surface using a clean rag and a mineral cleaner. Using a paint spray gun, apply fiberglass primer evenly across the area to be painted on the surface of the cap. Consult the primer's label for a designated drying time to allow the primer to set.
After the primer is completely dry, sand the surface by hand with 200-grit sand paper attached to a sanding block. Only remove a small amount of primer, and clean off any debris after you finish. Apply a second even coating of primer using the spray gun, and allow it to dry the same amount of time as the first coat. Sand again in the same manner, and wipe down the surface.
Clean out the spray gun nozzle before loading the fiberglass paint color into the gun. Spray a coat onto the surface, and allow the paint to dry. Wet a piece of sand paper, smooth the first coat, and wipe the area clean. Apply the last coat of paint to the surface, and allow it to dry. If additional coats are needed, repeat the sanding process before applying a new coat of paint. Use two to five coats of paint.