Hard water spots are caused by minerals like calcium and magnesium in your water. When a surface gets wet, the water evaporates but leaves the mineral deposits behind. Sprinklers, car washes, and showers can all cause hard water spots to form on glass. Chemically, these deposits are alkali, so they require an acid to remove them. With a few household ingredients, you can remove and prevent further mineral deposits.
Fill a squirt bottle with half water and half vinegar. The best vinegar to use is plain white distilled vinegar, which is inexpensive and effective. A higher concentration of vinegar may be even more effective, so adjust the vinegar to water ratio based on the severity of the water spots.
Spray the vinegar solution on the glass. Spray the solution on the glass, being sure to fully saturate any stubborn water spots or areas of concentrated buildup. Allow the vinegar to soak, and reapply a couple of times for particularly dirty areas.
Saturate a towel with the vinegar solution. Soak a rough towel in vinegar and water, and spread it over the sprayed area of glass. You can allow the towel to sit over the area so that the vinegar can soak in thoroughly.
Scrub the glass lightly with the towel. Use your rough, soaked towel to scrub the glass. You should not have to scrub hard. The bumps on the towel should act as an abrasive to help remove the spots and allow the vinegar to penetrate the mineral build-up.
Remove the vinegar solution with a dry towel or squeegee. You may want to spray the area once more with your vinegar solution or plain water, then towel off with a clean, dry towel (or paper towel) or use a squeegee to remove the water. Be sure you do not allow the vinegar solution or water to dry on the surface of the glass, as this will continue to leave more spots.