Clean the door using a scrub sponge and mild detergent. Even if the surface looks clean, chances are there's a substantial accumulation of furniture polish, dust and grime.
Fill dents, gouges and nail holes with carpenter's wood putty. Allow it to dry for at least one hour.
Sand the door with a flexible sanding sponge or 120-grit sandpaper until the entire surface looks dull and feels smooth when you run your hand over it. Brush or vacuum away sanding dust.
Prime the door with water- or solvent-based stain-blocking primer. Use a 2- or 3-inch brush for panel doors or a brush and 6-inch trim roller for flat doors.
Sand the door lightly after priming, especially if you've used water-based primer. This ensures a very smooth surface. Brush away the dust.
Apply caulk to gaps in panels or at joints by applying a bead of caulk and then smoothing it down immediately with your fingertip or a damp rag.
Apply two coats of satin or semi-gloss paint with a brush or roller.