Symptom
Hard-water spots
Mineral residue left on glass when tap water evaporates. Long-term exposure etches glass permanently.
White or clouded spots on glass caused by mineral-laden water (sprinkler overspray, rain running off a limestone roof, a hose rinse done wrong) evaporating and leaving dissolved minerals behind. Early-stage hard-water spots wipe off with a proper clean. Long-term exposure (5+ years of sprinkler overspray on a window) actually etches the glass surface — at that point the spot is permanent and only glass replacement or professional restoration can help.
Restoration uses an acid-based compound (typically oxalic acid or cerium oxide) with a polishing pad to remove mineral deposits before they etch. Etched glass is beyond restoration.