How do I get limescale off a shower screen without damaging the glass?

Why the usual answer works

Limescale is calcium carbonate left behind when hard water evaporates on the glass rather than being wiped off it. Acids convert carbonate into soluble salts, so the deposit dissolves and rinses away instead of having to be scraped. That is why every descaler sold is acidic, and why the vinegar advice is not wrong about the chemistry.

Usually recommended: Acetic acid (vinegar), Citric acid.

Why it must not be used here

The glass association's cleaning paper excludes acids from glass cleaning solutions as a class, naming hydrochloric and hydrofluoric as its examples, and warns that glass can be permanently damaged if it is infrequently or improperly cleaned. It also rules out the two things people escalate to when the acid does not work: abrasive cleaning materials, and razor blades or metal scrapers, which are excluded from routine cleaning altogether. The mechanism it gives for scratching is not the cleaner but trapped grit, so scrubbing a gritty screen is worse than leaving it.

This damage cannot be undone by cleaning. There is nothing left on the surface to remove.

More about glass and limescale.

What to do instead

Treat this as a drying problem rather than a cleaning one, because that is where the deposit is actually created. The glass paper is precise about the mechanism from the other direction: its pure-water method requires water below 20 ppm total dissolved solids and states that tap water is not acceptable, because the rinse water is allowed to evaporate in place. Ordinary tap water left to evaporate on a screen deposits exactly what it was carrying. So the intervention with evidence behind it is removing the water before it dries — squeegee or blot the screen after each use, which is the same instruction the sanitaryware manufacturer gives for metal, where allowing water to evaporate forms deposits and the technique is to blot with a dabbing action rather than rub. For scale already established, no supportable method exists on this substrate: the acid that would dissolve it is excluded by the glass authority, and abrasives and blades are excluded too.

• Rinsing and drying — The step people skip, and the one that decides whether cleaning protected the surface or damaged it. Residue left behind is exposure that continues after you stop.
• Blotting — Lifting a spill straight up into an absorbent cloth rather than working at it. The opposite of what almost everyone does, and the trade bodies are explicit about why.
• Shower screen — Glass that is rained on with hard water several times a day. The deposit is the problem and the usual solvent for it is an acid.

The chemistry behind both halves

• Water hardness — Dissolved calcium and magnesium picked up from the ground. It decides how much detergent you need, what your kettle looks like inside, and whether surfaces dry clean.
• Evaporative deposition — The mark is made when the water leaves, not while it sits. Everything dissolved in it stays behind on the surface.
• Etching versus staining — A stain sits on a surface and a mark from etching is a hole in it. They look similar, they need opposite responses, and cleaning harder makes one of them worse.
• Material compatibility — The reason a useful answer is about the surface rather than about the product. The same agent is the right answer and the wrong answer depending on what it lands on.