Polyurethane is a protective coating usually painted or sprayed onto wooden surfaces. It dries hard and clear and prevents wood from moisture and scratches. Polyurethane will dry on sink surfaces such as porcelain and fiberglass. If you used your sink to clean paintbrushes that you used with polyurethane and some of that polyurethane dried onto the surface of the sink, you can remove it with urethane strippers.
Apply a small amount of urethane stripper to a rag and then rub the surface of the polyurethane with the rag. The stripper resembles a thick gel and stays on the surface of the polyurethane.
Wait the specified amount of time printed on the label of the urethane stripper. The time will vary depending on the brand you use. Usually, 15 minutes is sufficient.
Scrape the surface of the polyurethane with a plastic scraper to remove the loosened urethane and to remove the stripper.
Apply more stripper to the remaining urethane and repeat the process. Do this until all of the urethane is gone.
Clean the sink with warm water and a clean rag to remove the urethane stripper.