Mineral salts may be present in rainwater or condensation that forms inside the house, making the water hard. When that hard water collects on windows and evaporates, white mineral stains may be left behind, and you usually can't remove these with window cleaners or soap and water. You need an acid to dissolve these salts or a fine abrasive that won't damage glass to rub them off. Household cleaners that contain oxalic acid work, but you can do the job with vinegar, lemon juice or white toothpaste.
Dissolve Mineral Stains with Vinegar or Lemon Juice
Mix a 50-50 solution of white distilled vinegar and pour some into a spray bottle. Alternatively, mix lemon juice with water in the same proportion and pour the mixture into the bottle. Vinegar contains acetic acid and lemon juice contains citric acid. Both acids dissolve mineral salts.
Spray the cleaning mixture on the mineral deposits and allow it to remain for about 10 minutes. You should see the salt deposits noticeably dissolving. Refresh the solution if it dries out.
Rub the cleaning solution off with s soft cloth. Repeat the treatment if any discoloration remains.
Spray the window with a conventional window cleaner and squeegee it clean.
Rub Out Stains with Toothpaste
Squeeze a generous amount of white toothpaste on a soft rag.
Wet the cloth with a little water and vigorously rub the spots until the stains are gone.
Spray the window with window cleaner and squeegee it clean.
Cola also contains an acid -- phosphoric acid -- that can dissolve hard-water stains. If you have to clean severely stained glass in a construction situation, you may prefer muriatic acid to weaker ones -- but be sure to wear gloves and goggles if you choose this option. Baking soda is a mild abrasive that removes stains and neutralizes acids, so rubbing it on a difficult stain is a good follow-up to an acid treatment.