Polyurethane can be used over enamel paint, which often provides a harder finish than latex-based paints. For best results enamel paint should be allowed to dry for three to seven days before applying the finish.
Failure to allow proper drying time can lead to the paint chipping or peeling, and may reduce the effectiveness of the topcoat. After the paint has completely cured, a thin coat of polyurethane should be applied with a dense paintbrush or foam pad. This coat should be allowed to dry for two hours then lightly sanded and wiped clean. For best results this process should be repeated three or four times.