Glass shows hard water more clearly than any other surface, and it is also the surface most easily ruined by trying too hard. The deposits are mineral and dissolve in mild acid given contact time. Abrasives remove them faster and leave fine scratches that hold the next deposit more tightly, which is why glass that has been scrubbed gets worse every year.
Dissolve rather than scrub
The USGS describes hardness as dissolved calcium and magnesium, and what you are looking at is those minerals left behind as water evaporated. White vinegar or a commercial descaler applied wet and left in contact is the method, and contact time does the work: a cloth soaked and laid against a shower screen, or a spray reapplied so the surface never dries, achieves far more than pressure. Repeat rather than escalate, since heavy build up may need several cycles.
Etching, and why some marks will not come off
Deposits that have sat for years can etch the glass surface, at which point the mark is damage rather than a deposit and no cleaner removes it. The test is simple: if the surface still feels rough or looks cloudy after the deposit has clearly dissolved, it is etched. Polishing compounds intended for glass can improve mild etching, and severe cases are a replacement. This is the reason to keep on top of screens rather than tackling them once a year.
Glassware from the dishwasher
Cloudy glassware has two causes that look identical and need different responses. A film that dissolves in vinegar is hard water deposit, and rinse aid plus a descaling cycle usually clears it. A cloudiness that does not dissolve is etching, caused over time by soft water combined with too much detergent and high temperatures, and it is permanent. Working out which you have with a vinegar soak decides whether to change the detergent dose or accept the glasses are finished.
Preventing it
Drying the surface before the water evaporates leaves nothing behind, which is why a squeegee after each shower keeps a screen clear for years. On windows, avoid sprinklers throwing onto the glass. And in the hard and very hard bands, where deposits form fast enough that no routine keeps up, softening the water is the durable answer since it removes what would otherwise be left behind. Our hard water shower head guide covers the point of use options.
Questions people ask about hard water stains on glass
What removes hard water stains from glass?
Mild acid with contact time: white vinegar or a commercial descaler kept wet against the surface, repeated rather than scrubbed.
Why will some marks not come off?
Long standing deposits can etch the glass, which is damage rather than deposit. If it still feels rough after dissolving, it is etched.
Why is my glassware cloudy?
If a vinegar soak clears it, it is hard water deposit. If it does not, it is etching from detergent and heat, which is permanent.
How do I stop it coming back?
Squeegee surfaces before the water evaporates, and in the hard bands soften the water so nothing is left behind.