Things You'll Need
Professional painter's tape
Canvas drop cloths
Wooden stir stick
2- to 3-inch polyester paintbrush
A polyurethane finish can prolong the life of paint and prevent ultra violet rays from causing it to fade. Unfortunately, when many do-it-yourselfers apply polyurethane over a water-based latex or acrylic paint, they end up with rampant flaking. If you want to avoid a similar outcome, choose a specific type of polyurethane, specifically formulated to bond with water-based finishes. Remove oils and foreign particles from the painted finish, before you apply the sealer, or the finish may fail.
Wash exterior painted finishes, using a pressure washer. Intense water pressure may strip the paint. Avoid this by using a low-pressure, wide-angle spray tip. Rinse interior painted finishes, using wet rags. Let the painted surface dry for four hours.
Wipe down the painted finish, using tack cloths.
Cover surfaces adjacent to the painted finish, using a low-tack tape. Spread drop cloths out beneath the finished surface.
Open the can of polyurethane, then stir the clear finish, using a wooden stirring stick.
Apply the polyurethane to the painted finish, using a 2- to 3-inch polyester paintbrush. Brush left to right, applying gentle pressure as you move your paintbrush.
Look closely at the polyurethane finish to see if it is sagging. Quickly smooth these types of flaws before they have a chance to fully dry. Let the polyurethane dry for two hours.
Oil-based polyurethane will not bond to water-based paint. Be sure to select a compatible water-based polyurethane finish, or flaking will result.