Polyurethane is a clear finish that can be brushed or sprayed onto wood with ease as a top coat finish. It is important to carefully sand the wood surface with a progression of sandpapers until the surface is completely smooth. You should apply at least two coats of polyurethane for a durable finish and sand between coats to ensure a smooth finished surface. Because polyurethane breaks down in sunlight, it should not be used for exterior projects. It is, however, ideal for finishing interior projects with a clear coat.
Sand the surface where you plan to apply polyurethane with a progression of sandpaper starting at 80-grit and finishing at 180-grit for a completely smooth surface. Always sand with the grain of the wood.
Wipe off the surface of the wood with a tack cloth to clean it just prior to applying the polyurethane. It is important to have a dust-free surface for a smooth finish that adheres sufficiently.
Brush on an even coat of polyurethane with a polyurethane brush in long smooth strokes. To remove any air bubbles that develop, lightly back-brush those spots and smooth out the finish. Allow the first coat to dry at least four hours.
Sand the finish with 280-grit sandpaper. Wipe off the surface with a tack cloth. Apply a second coat of polyurethane in the same manner as in Step 3.
You can apply a third coat if desired for a more durable finish.
Wear eye protection when sanding. Apply polyurethane in a well-ventilated area.