Things You'll Need
Polyurethane is a clear top coating applied over wood finish and paint. It is a thick substance that is slightly more complex to work with compared to paint as any imperfection during application is emphasized once the coat dries. Since it is impossible to remove polyurethane from a brush using only water, finding an alternative cleaning method is a necessity for cleaning brushes.
Fill the plastic bowl halfway with paint thinner. The paint thinner need only be applied to the bristles, or foam section, of the brush in order to remove the polyurethane.
Dip the brush tip into the bowl of paint thinner. Cover the entire brush end with paint thinner. Add more paint thinner to the bowl if needed.
Leave the brush in the bowl of paint thinner for 15 to 20 minutes. Allow the paint thinner to fully penetrate the polyurethane within the brush.
Remove the brush from the bowl and rinse it with lukewarm water. Do not use cold water as this can cause the brush to harden. Avoid using extremely hot water as well as this can cause the glue securing the bristles to loosen and the bristles to fall out.
Pat the brush dry with an old towel.