Have you ever tried to remove sun-baked water spots from your window glass? Did you have any luck? Aside from the minor mineral deposits a stray sprinkler may leave on glass, the more insidious effect is actually an etching action resulting from the mineral-laden water drying in direct sunlight. Neither solvent, nor acid nor ammonia-based window cleaner will touch this damage. There is only one way to remove these spots—polish them off.