Hard-water deposits are alkaline, which means that an acid cleaner is the best approach for cleaning them. Phosphoric acid works well and is safe for most surfaces. If you buy cleaners containing phosphoric acid in a grocery store, it will likely contain about 4 percent to 6 percent acid. You can purchase lime scale removers at janitorial supply stores that contain 8 percent to 12 percent acid, which will get the job done faster. A higher concentration of acid is safe on most household surfaces as long as you rinse the surface to remove all traces of the acid once the cleaning is complete. You should let acid sit for a few minutes after you apply it to let it work. Tough hard-water deposits may take more than one application. You can scrub the applied areas with a white, nylon-backed scrub sponge. Make sure you read any manufacturer's warnings before applying phosphoric acid solutions to surfaces in your home.