If you would like to paint a fibreglass garage door, you will need to employ specific preparation and application strategies. First, because fibreglass is nonporous, you have top rough it up it to promote adhesion, or the finish will peel. In addition, because fibreglass is so smooth, you will need to apply the paint in a particular way that will promote a professional-looking finish, free of sagging and brushstrokes.
Clean the fibreglass garage door using a pressure washer. Begin the washer on a low setting, and gradually increase intensity. Allow the garage door to dry.
Abrade the fibreglass to promote adhesion by sanding it with a palm sander stocked with a fine-grit sandpaper. Stop sanding when the garage door feels slightly rough.
Cover parts of the garage door you do not want painted with blue painter's tape. Open the garage door, and slide heavy-duty fabric dust sheets underneath it. Close the garage door.
Apply a light coat of acrylic latex primer to a 90 cm (3 foot) wide section of the garage door using the roller. Smooth the wet primer using a 10 cm (4 inch) paintbrush manufactured for latex paints. Continue the application process until the fibreglass is completely coated with primer. Raise the garage door about 8 cm (a few inches) until the seams open slightly. Allow the door to dry for two hours.
Wash the paintbrush with plain water.
Apply a coat of exterior acrylic latex paint in the same way you applied the primer. Open the garage door a few inches until the seams open slightly. Allow the door to dry for two hours.
Be sure to raise the garage door about 8 cm (a few inches) until the seams open slightly, or the garage door could stick shut.
Do not begin the pressure washer on a high setting, or you could damage the fibreglass garage door. Do not use sandpaper with a grit lower than 300, or you could mar the fibreglass with scratches that could show through in the final finish.