Combine 1/3 cup trisodium phosphate with 1 gallon water in a bucket. Dip a scrub brush in the mixture and shake it off. Scrub down the door vigorously. Wipe down the wet door with an old towel and allow it to air dry for an hour.
Press a piece of 80-grit sandpaper onto the bottom of an electric sander. Run the sander along the surface area of the door thoroughly. Repeat with 200-grit sandpaper and 300-grit sandpaper. Swab down the door with a tack cloth, removing all dust.
Dip a paintbrush into the stain of your choice. Cover the door with a thick coat of stain, applying the strokes in even, up and down movements. Allow it to soak for 20 minutes and then wipe off the excess with a lint-free cloth. Repeat two more times.
Dip a fresh paintbrush in a clear polyurethane that is designed for outdoor spaces and is waterproof and UV-resistant. Cover the door with a light coat of the polyurethane, allowing it 24 hours to dry. Repeat with a second coat.