If nothing else works to remove a urethane stain from an oxidized painted surface, lacquer thinner can be used. Lacquer thinner is a strong solvent that can damage the paint if it is not used carefully. Always test lacquer thinner on a small, inconspicuous area before using it on a large area. To use lacquer thinner, dampen a cloth with the solvent and rub it over the urethane stain. Work in small sections and do not rub too hard. Rinse the area with water after you have removed the stain.