One way to stain a fiberglass door involves cleaning the door and painting it with gel stain followed by a urethane topcoat. You need clean rags, two stirring rods, painter's tape, screwdrivers and a urethane topcoat to complete this task. Mineral spirits, gel stain and natural-bristle paintbrushes are also required.
For an already-installed door, start by detaching the door from the frame. To do this, remove the pins that attach the door to the hinges, and lift the door from its place. Using a screwdriver, loosen off the screws attaching the hardware to the door, place the hardware aside, and rest the door on a work surface. Be sure to work in a room with proper ventilation. Additionally, keep the stain and topcoat from the work surface by covering it.
Cleaning the door involves wiping it with a clean rag dampened with mineral spirits to remove dirt and grime and enhance stain bonding. Allowing the door to dry completes this process. Before applying the stain, tape the door sections you don't intend to stain. Remove the lid from the gel stain can, stir the stain to a fine mixture, paint the stain over the door in an even layer, and allow the stain to dry according to the manufacturer's instructions. Once the surface is dry, stir the urethane topcoat, and paint the stained surface with the coat using another brush. Allow the coat to dry, and repeat the entire process for the other side of the door before replacing the hardware and reattaching the door to the frame.