The cleaning is not the risky part. The tool is.
Scale is calcium and magnesium carbonate, and carbonate dissolves in acid — that is the whole chemistry and it is not complicated. What makes this job worth a page is that a tap is a thin chrome film over brass, and the things people use to get scale off are the things that go through that film. A scouring pad cuts it. Hydrochloric acid — the active ingredient in most toilet descalers — eats it. Chlorine bleach pits stainless steel. Boiling water attacks the rubber seals inside the cartridge, which then leak. Every one of those is a bigger repair than the scale was, and none of them is necessary, because the scale comes off a soft cloth and a mild acid given twenty minutes.
So the working rule for this page is: acid is right, aggression is wrong. Citric acid is the one to use — it is strong enough to dissolve hard scale at 200 g/L in warm water, mild enough that the tap manufacturers who forbid vinegar on plated finishes still recommend it, and it does not smell like a chip shop. Vinegar works too on light scale and ordinary finishes, at about one part to one of water, but it is acetic acid and some high-end plated surfaces do not tolerate it. And whatever you use, put it on the cloth rather than spraying it at the tap: liquid sprayed at a tap runs into the body and sits against the seals.
One sentence, thirty seconds, and the first two faults are separated
Unscrew the aerator and run the tap again. That is the test, and it costs nothing.
- The flow immediately gets strong and straight. The scale was in the aerator — the little screwed-on end piece with two or three mesh layers in it. This is the common case by a wide margin, it is free to fix, and if the mesh is split you can replace the whole aerator for less than the price of a coffee.
- It is still weak. Then the restriction is inside the body or the cartridge, and no amount of work on the end piece will help. Different repair, further down this page.
- The flow was never the problem — there are just white marks on the chrome. That is cosmetic, it is the easiest of the three, and it is also the one where people do the most damage, because a white mark invites scrubbing and scrubbing takes the finish off.
The aerator itself is worth knowing properly. Its mesh is fine — holes of half to eight-tenths of a millimetre — so it collects grit, rust flakes and scale fragments, and the colour of what comes out tells you about the supply: reddish-brown flakes are rust and mean the pipework is corroding somewhere upstream, white hard lumps are the scale you expected, and yellow-brown silt is sand coming in from outside. A healthy aerator makes the water come out milky and half-transparent, quietly, without splashing; a clear jet that splashes means the air-mixing path inside it is still blocked. And when you take it off, put a bowl underneath — there is a silicone washer in there that drops out and is never seen again, and a tap that drips at the spout after reassembly is usually that washer, not the threads.