A clear polyurethane sealer can add a glossy finish to painted wood and protect the finish from premature wear. Unfortunately, when eager amateurs try to add polyurethane over the top of a painted finish, they frequently end up with noticeable peeling after only a few hours or days. This usually occurs because they fail to understand the importance of compatibility. While petroleum-based polyurethane sealers are well-suited for oil-based finishes, such as stains and enamels, they will not bond to water-based acrylic and latex paint. Choose the proper type of sealer based on the type of paint you're working with, or finish failure will result.
Rinse exterior painted wood, using a pressure washer. Be sure to equip the washer with a wide-angle tip that won't strip off the existing paint. Wipe down interior water-based paint, using wet rags; wipe interior oil-based paint, using a rag dampened with clean white spirit solvent. Let each surface dry for two hours.
Attach painter's tape to any surface running adjacent to the painted wood.
Open the container of polyurethane and stir the clear sealer, using a wood stir stick.
Apply the correct type of polyurethane to the paint, using an appropriate paintbrush. Add water-based poly to existing water-based paint, using a polyester brush; apply solvent-based poly to existing oil-based paint, using a natural-bristled paintbrush. Apply gentle pressure with the brush and carefully smooth sags and/or runs before they have an opportunity to solidify.
Let the polyurethane finish dry for two hours. Wash polyester brushes with tap water; wash natural brushes with white spirit solvent.
Don't apply water-based polyurethane over existing oil-based paint, or peeling will follow; likewise, don't add oil-based poly over the top of existing water-based paint.