Hmmmmmm.......curious. I have the same problem with our fiberglass tubs. Also the dreaded toilet ring. It is my fault for not cleaning often enough to prevent. Nothing available in the bathroom cleaning section at the wal-marts sends to have enough deadly chemicals in out to take the stuff off.

I am sure my grandma would have had a trick up her sleeve....but she also had time to clean....no cell phone, no internet, no tv, no RC forums....