Glad you asked...
A fiberglass truck cap is painted, like an automobile or boat, using an enamel based paint. You will need to fill and sand imperfections before you apply paint to the cap.
Then again, this answer might help you...
Paint a fiberglass truck cap after cleaning the cap with acetone to make sure the paint sticks well. Use a gel coat and hardener to spray on the cap. After the gel coat is done, apply the paint fast so it bonds with the gel coat. For more information look here: http://www.ehow.com/how_5619857_paint-fi...;

