There are a few things you can do to clean a polyurethane brush before using it again.

  1. Soak the brush in warm water for a few minutes. This will help remove any built-up dirt and debris that may have built up on the bristles over time.
  2. Use a mild soap or detergent to clean the bristles of the brush. Be sure to use enough soap or detergent to cover all of the bristles, and be gentle when cleaning them.
  3. Dry the brush completely by shaking off any excess water or soap, and then place it back in its packaging.