In my recently acquired house/pool some of the plaster seems stained or dirty. Scrubbing doesn't seem to do anything for it, though there seem to be scrub marks in what I think is the mottling. So perhaps HARD scrubbing works, I'm not sure.
I want to get rid of it, I asked a guy I know that cares for pools and he said the only way is to drain and acid wash the plaster. There's gotta be a better solution?