Acid washing destroys pool surface, removing the stain, but leaving the plaster rougher, thinner, more porous, and thus more susceptible to stains!
Try using Iron Out -- you can get a bottle at Walmart in the detergent section -- and see if that helps. If Iron Out doesn't remove, or at least lighten it, it's probably NOT a metal stain.
If Iron Out does work, you may want to get a larger quantity from Amazon:Super Iron Out IO30N Rust Stain Remover, 30 ozThe larger size ships from Amazon, so if you get the 1 month Amazon Prime trial, you can have it shipped 2-day for no extra charge.
Super Iron Out IO65N Rust Stain Remover, 65 oz
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