I actually buy this product to use on the inside of the shower doors in my house. We have water that is so hard your don't need a glass! I clean the shower enclosure glass completely - including scraping the limestone off with a razor blade - then I apply Rain-X as directed. It really helps keep the buildup to a minimum. Each treatment lasts about 6 weeks if I squeegee the glass after each shower but sometimes there is no time and the interval is reduced.