Remove the locking mechanism, door knobs and hinges. Set them to the side. Lay the wooden door on a pair of sawhorses.
Sand the doors lightly with sandpaper to remove the rough edges. Sand away rough spots by pressing down lightly with the sandpaper and sanding in circular motions.
Dust the sand away with a tack cloth. Apply wood putty to any knots. Smooth the wood putty over the knots with a putty knife. Allow the putty to dry for one or more hour. Sand over the knots with sandpaper. Dust away the debris with a tack cloth.
Paint the door with a laytex primer, smoothing it on with a paintbrush. Allow the paint to dry and then sand the door surface lightly with sandpaper. Wipe away the debris.
Paint on two coats of white interior paint using a paint brush. Apply the paint with smooth motions of the wrist. Allow the paint to dry between coats.