Polyurethane has replaced many long-trusted coatings due to its durability and ease of use. It adds a deep shine and a long-lasting protective barrier that keeps furniture, counter tops and wood floors beautiful for years. It can be applied to raw wood, stained wood or painted surfaces, but some care must be taken when applying to painted surfaces, especially if the color coat is oil based.
Make sure the surface is clean and dry. Polyurethane will seal in dirt, dust and pet hair and preserve it forever in the final finish. Determine if the oil-based color coat is worth keeping. If it is badly damaged or if you no longer have any of the paint for touch up or repainting, consider sanding and priming with an oil-based primer and repainting with water-based paint.
Paint a fresh coat of oil-based color if you still have it, and contact the manufacturer about the "recoat window." This means you will want to apply the poly when the paint is dry enough to recoat but not yet fully dry for everyday use. This will promote adhesion.
Scuff sand the color coat if you no longer have the original paint or if you have let the fresh coat dry completely and missed the recoat window.
Add a thin barrier coat of shellac if you are concerned about adhesion or scratches left from sanding.
Use a natural bristle brush for oil-based polyurethane and a synthetic bristle brush for water-based polyurethane. Dip the brush directly into the can and do not wipe it on the can's edge. That causes air bubbles to show up in the finish.