To paint an exterior fiberglass door, start by thoroughly cleaning the door's surface. Using painter's tape, cover any accessories not to be painted such as doorknobs, windows or door frames. Apply bonding primer to the door and allow two hours to dry. After the primer dies, mix the desired paint, making sure there are no air bubbles. Paint the door in sections, allow it to dry and finish with a sealant.
Clean the door with a layer of acetone or mineral spirits. When the odor has dissipated, go over the door's surface again with a mixture of water and either dish soap or laundry detergent. Both of these cleaning agents gently and effectively cut through grease. Rinse the door with a hose and dry it well. To test if the primer has sufficiently dried, visually check it to make sure it is no longer tacky or overly thick.
When painting the fiberglass door, paint with the grain of the textured surface. Paint in smooth strokes with a high quality brush. If painting in sections, lightly feather the brush strokes when meeting previously painted portions of the door. After the paint dries and a sealant is applied, remove the painter's tape.