Remove the door from its hinges. Lay it flat on a pair of saw horses or other sturdy work surface. Examine the stiles, which are the vertical outside pieces, and the rails, which are the horizontal pieces, to make sure all of the joints are tight. Inject wood glue into loose joints, using a syringe. Drill a 1/16-inch hole if needed to give the syringe access. Press the loose pieces into proper alignment, using bar clamps, and leave the clamps in place for at least three hours.
Apply a coat of gel stripper to the face of the door. If you do not intend to refinish the inside face, run masking tape around the edge of the door and allow it to stand up ¼ inch above the face of the door to prevent stripper from dripping onto the inside face. Allow the label recommended working time for the stripper.
Scrape the stripper and old finish from the door, using a chemical-resistant plastic scraper. Use quad zero steel wool to remove the finish in detailed areas. Wipe the remaining stripper from the door with a rag dampened in mineral spirits. Wear gloves, especially if you have sensitive skin.
Fill any cracks or small gaps in the door face with solvent-based, oak-colored wood filler. Allow it to harden for 10 to 15 minuted before sanding.
Sand the face of the door with 100-, 150- and 200-grit sandpaper in successive passes. Use a random orbit sander for best results. Always move in the direction of the grain when sanding. Brush the dust from the surface of the door with a clean, dry paintbrush. Strip and sand the other side and edges of the door if you intend to refinish them, using the same techniques.
Apply a coat of stain in your choice of colors. Use water- or oil-based stains. Use a fine, soft-bristle brush and work in the direction of the grain for best results. Wipe excess stain from the surface of the door with a clean, dry rag. Allow the stain to dry according to label recommendations. Lightly sand the door with 220-grit paper to smooth any grain that may have been raised in the staining process. Brush dust off with a dry, soft paintbrush.
Apply at least two coats of clear finish. Use oil-based finish for oil-based stains and either water- or oil-based finish for water-based stains. Allow label recommended drying times between coats and before rehanging the door on its hinges.