Successful painting of wood moulding lies in the preparation. New moulding is usually sold bare and needs protection from moisture. Used moulding may have an existing coat of paint or stain and needs to be properly sanded or covered lest it bleed through the finish.
Prepare, Prime, Paint
Preparation involves making sure the moulding is free of splinters, snags and other inconsistencies. Go over new moulding with medium-grit, followed by fine-grit sandpaper to ensure it's smooth. Use an air hose or slightly damp cloth to remove all dust particles. Apply a latex primer to the moulding. If it's used moulding with existing paint, additional sanding may be needed. Prime stained moulding with a stain-blocking primer or shellac. Once dry, apply 1 to 3 coats of your chosen paint. You need only prime and paint the parts of the moulding that will be exposed.