Hard water stains on glass can usually be removed with vinegar, warm water, baking soda and a mild dishwashing detergent. More persistent stains on large glass surfaces such as shower doors may need to be treated with commercial products specially designed for hard water deposits and rust stains.
To clean glassware, fill a dishpan or sink with equal parts vinegar and warm water. Add the glassware, let it soak for an hour, and then wash it with a mild dishwashing liquid. Dry the glassware with a soft hand towel or microfiber cloth to avoid spotting. If deposits remain, fill a dishpan or sink with warm water, and mix in 2 to 3 tablespoons of liquid Calgon. Soak the glassware for at least an hour, and then wash and dry it as before.
To clean shower doors and other large surfaces, make a paste of vinegar and baking soda, and use a sponge to apply it to the hard water stains. Leave the past for 15 minutes, and wipe it away with a damp towel. Use a squeegee to remove any excess moisture. Repeat if necessary, or use a commercial product designed for removing hard water stains, and follow the instructions on the label.