Determine what is wrong with the chair molding. Most often, chair rail molding needs to be repaired when it is dented, chipped or cracked.
Fill large dents or cracks in the chair rail molding with wood putty. Scoop the putty out of the container with a putty knife and spread it over the damaged area. Let the putty dry per manufacturer directions, or overnight. If the chair rail has a piece that has broken off, apply a small amount of wood glue to the broken edge and stick it back into place. Tape it in place with painter's tape and allow the glue to dry.
Sand the damaged area of the chair rail molding with fine grit sandpaper. This should smooth the area so that it matches the rest of the chair rail molding. Wipe away the dust from the molding with a damp cloth.
Prime the damaged area on the chair rail molding using latex painting primer and a paintbrush. Apply the primer with the paintbrush and let it dry per manufacturer directions.
Paint the damaged area on the chair rail molding using leftover paint from the chair rail, or match the paint as well as possible. Let the paint dry per manufacturer directions.