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Hard-water deposits are alkaline, so an acid-based cleaner is 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 janitorial supply stores that contain 8 percent to 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 the surface to remove all traces of the acid after the cleaning is
complete. Let the acid sit for a few minutes after you apply it to let it work.
Tough hard-water deposits may take more than one application. 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.