Use Bar Keeper's Friend to clean Spots and Hard Water Stains on a Glass Shower Door. Bar Keepers Friend traditionally comes in a powdered can (like Comet). Their “multipurpose cooktop cleaner,” in thick liquid form, is easier to use. IMPORTANT note: never use to remove soap scum from stone, like marble. The acidic properties that remove the scum will also degrade the finish on your stone. Opt for ammonia instead.