How to Fix a Non-Smooth Surface With Polyurethane

Mix an oil-based polyurethane with an equal amount of paint thinner for the first coat. This thin coat fills small scratches and blemishes to provide a smooth base for future applications.

Paint the thinned polyurethane onto the wood with a 2-inch wide bristle or foam brush. Apply it in the same direction as the wood grain, using long, even strokes. If bubbles form, run the brush back in the opposite direction to pop and smooth them out.

Allow the first coat of polyurethane to dry for 24 hours. Sand the surface with a 280-grit sandpaper until the finish feels smooth to the touch. Vacuum the sanding dust from the surface.

Apply a second coat of pure, unthinned polyurethane. Paint it on using the same method previously, applying it as thinly as possible. As you paint, brush over any runs or bubbles that form, to minimize imperfections later.

Sand the wood a second time and apply a third coat, following the same procedures. Most surfaces require three to four coats to ensure a perfectly smooth surface.

Inspect the surface for blemishes after 12 hours and after applying the final coat. Hold a razor blade flat against any polyurethane runs or bubbles and carefully cut them off flush with the surrounding surface. Apply a thin coat of thinned polyurethane over the repairs, blending it in with the brush.