Hard-water deposits are alkaline or calcium, so an acid-based cleaners are the best way to clean them. Phosphoric acid works well and is safe for most surfaces. Grocery store cleansers with phosphoric acid contain 4 percent to 6 percent acid. You can purchase lime scale removers at any janitorial supply stores which contain 8 percent to about 12 percent acid to get the job done faster. A higher concentration of acid is safe on most household surfaces as long as you rinse thoroughly to remove all traces of the acid. (Let the acid sit for 3 to 5 minutes to give it time to work before scrubbing the surface in need of cleaning.)
Tougher deposits may take more than one application and extra scrubbing. Scrub the areas with non-scratch nylon-backed sponge.
Before applying phosphoric acid solution to any surface in your home, making sure you read the manufacturer's warning label.