Painting a fiberglass entry door is much like painting a wood door, without a need for sanding before paint is applied. Use the type of primer and paint recommended by the door manufacturer. Some doors don't need primer, but typically a good-quality primer helps the topcoats of paint adhere better and creates a longer-lasting finish. Paint both sides and all edges of the door for maximum protection from moisture and changing weather conditions and to help prevent warping.
Remove all weatherstripping from the door, following the manufacturer's directions. Remove loose dust and debris by brushing down the door with a soft-bristled brush. Wipe it with a soft, dry cloth to remove additional dirt.
Wash the door with a few squirts of dish soap added to a bucket of warm water. Dip in a sponge and wipe down the entire door, starting at the top. Wash the top edge as well, as dust tends to accumulate atop a door if it is left open. Rinse the door by wiping it down with a damp sponge, then allow the door to dry completely.
Cover hardware, window glass and areas surrounding the door, such as the doorjamb, with painter's tape to protect it. Cover the floor of the entryway and area just beyond with newspaper if you are painting the door in place.
Ventilate the work area. Open and stir the primer with a stir stick, pouring some of it into a paint tray.
Dip a synthetic brush into the primer, wiping excess off onto the inside edge of the paint tray. Apply the primer to the edges of the door, including the bottom and top edges.
Prime the exterior side of the door, following the faux grain pattern if the door has a wood-grain finish. The grain texture typically runs vertically on the vertical portions of the door and horizontally along the top and bottom and beneath or above any windows. Avoid applying the primer too thickly, as this muddles the detail and profiles of the door elements. Allow the primer to dry as directed.
Apply a light, even coat of paint using the brush, and let the paint dry as directed. Apply additional coats as needed, allowing the paint to dry between coats.
Apply painter's tape around the edge of the door once the exterior paint has cured completely. Prime and paint the interior surfaces of the door as before, using a good-quality interior primer and paint.