Oil based polyurethane brushes are a popular choice for many artists because of their smooth, even surface. However, they can be difficult to clean. Here are some tips to help clean an oil based polyurethane brush:

  1. Use a mild soap and warm water to clean the brush.
  2. Rinse the brush in cold water and then dry it off.
  3. Use a vacuum cleaner to suck the dirt and oil out of the bristles.
  4. Place the brush in a plastic bag and store it in a cool place until you need it again.