If you want to finish a fibreglass door with a fresh coat of paint, you'll have to focus heavily on preparation, or you won't be able to achieve adequate adhesion. Because fibreglass doors are poorly suited for primer and paint adhesion, they require special treatment before application. Without this treatment, the doors will shed their new finish shortly after application. Once the door is prepared, it will accept paint adhesion. However, you should employ a particular application method, or you may end up with an uneven-looking finish, marred with unsightly brushstrokes.
Open the fibreglass exterior door. Remove hardware from the door, using a screwdriver.
Abrade the fibreglass exterior door to promote surface adhesion by sanding it with 220-grit sandpaper.
Slide sheets of masking paper underneath the door. Tape the edges of the paper down with a low-tack blue painter's tape.
Fill a five-gallon bucket halfway with the primer. Apply a coat of acrylic latex primer to the abraded fibreglass exterior door, using a roller. Smooth the wet primer, using a four-inch paintbrush, manufactured specifically for use with latex paints. Wait two hours for the primed fibreglass exterior door to dry.
Clean your painting tools with water.
Coat the primed fibreglass exterior door with acrylic latex paint. Apply paint to the door just as you did the primer. Use a new nap roller cover.