The Acidic Solution
Hard water stains are alkaline, so you need an acid to dissolve them. White vinegar or lemon juice works wonders.
Cleaning Faucets
Soak a paper towel in vinegar and wrap it around the faucet. Secure it with a rubber band if needed. Let it sit for an hour. The vinegar will break down the mineral deposits, allowing you to wipe them away easily.
Cleaning Shower Doors
Mix equal parts vinegar and water in a spray bottle. Spray the glass thoroughly and let it sit for 15 minutes. Scrub with a non-abrasive sponge and rinse. For tough spots, use a paste of baking soda and vinegar.
Preventing Buildup
The best way to deal with hard water stains is to prevent them. Wipe down wet surfaces after use. Installing a water softener can also solve the problem at the source.