Couple of ways:
Vinegar and baking soda into a loose paste. Apply it on and let it sit for like 5 minutes. Rub off with newspaper.
Vinegar, lemon juice and mild dish soap. Spray on and let sit for a few minutes. Rinse off with distilled water and dry with paper towels or newspaper.
Vinegar and distilled water for light spots. If the spots are not that caked on try that first. It's a safe alternative to windex.
Razor blade for stubborn spots.
The thing is those spots are alkaline mineral deposits so attacking them with acid will help dissolve it.
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