Place the tabletop in a well-ventilated, dust-free environment.
Sand the tabletop several times, using successively finer grades of sandpaper until the surface is smooth and unblemished. Wipe the tabletop free of dust with a tack cloth between each application of sandpaper. Start with 100-grit sandpaper, then 150-grit and 220-grit for the final application.
Thin the polyurethane coating with mineral spirits at a ratio of 2 parts polyurethane to 1 part mineral spirits. Apply a coat of the mixture with a paintbrush and let it dry completely. Make sure the surface is hard and not tacky.
Sand the surface with 220-grit sandpaper and wipe with a tack cloth.
Apply two more coats of polyurethane with a brush, letting the polyurethane dry completely between applications.