Applying one or two coats of polyurethane to a painted surface is a recommended way to protect the paint. Use a water-based product if you're concerned about color because it won't yellow as much as an oil-based one. Oil-based polyurethane levels out to a smoother finish, although it takes several hours longer to dry. You can apply polyurethane over any type of paint, as long as it's clean and has been properly prepared.
Preparation and Application Procedure
If you're coating fresh paint, allow the paint to cure first. Curing times vary according to paint type and atmospheric conditions, but a safe average is about a week.
If the paint isn't fresh, wash off grease or stains with a strong detergent solution. Mix 1/2 cup of trisodium phosphate with a gallon of warm water and scrub with a sponge or rag. This deglosses the finish as well as cleans it, providing for better polyurethane adhesion.
Let the paint dry, then scuff with 120-grit sandpaper to etch the surface. The goal is to slightly flatten the sheen without producing deep scratches that might be visible under the polyurethane. You can do it by hand, or use a palm sander. If you're applying polyurethane to a painted floor, scuff the floor with a floor buffer and a 120-grit sanding screen. Wipe the sanding dust away with a damp rag or tack cloth.
Apply oil- or water-based polyurethane with a brush, or spray it. Avoid using a roller -- it leaves bubbles that may harden into the finish. Do not thin the polyurethane before spraying it -- it's thin enough to spray out of the can, and adding more thinner may cause it to run.
Let the first coat dry for the time recommended on the container, then scuff it lightly with 120-grit sandpaper. Scuffing removes small dust particles as well as flattening any bubbles that hardened into the finish.
You shouldn't need any more than two coats. Let the final coat dry for the recommended time, then give it an extra 24 hours to harden before walking on or handling the object you painted.