OK. Different info: "stains are everywhere" and not just the patches. Plus I could see a little better in one of your pictures (cropped, rotated, and enhanced below).

It looks to me like what's sometimes called "plaster mottling". Whether it's a defect or an inherent characteristic of modern pool plaster (which is missing some ingredients used 40 years ago, such as asbestos) depends on who you talk to -- but I can tell you it's quite common. (There's another sort of "mottling" that is more spots than streaks, that has other causes, and is a defect. But in that case, the argument is, is it a defect in the plaster or a result of water chemistry.)

Anyhow, the only fix for the first type of mottling is a replaster job - but it may reappear. Acid washing may reduce it, but at the cost of a rougher and more porous surface.

I could be wrong, however, so DO try the cal hypo.



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