I have a rust stain on the pool ledge about 2 feet under waterline... roughly in the shape of a 2ft x 1ft rectangle. It's a plaster pool with the pebbly looking surface.
Fortunately I know what caused it. Some idiot used a Dremel tool to cut off the anchor bolts he used for the pool slide and got a bunch of metal shavings in the pool.
I think I've got the metal shavings out, but the stain won't come clean from using a scrub brush.
Best idea I have come up with so far is to use a large diam pvc pipe or maybe some sonotube to surround the stained spot and come above the water line. Then I could pour some acid in it to dissolve the stain. Then pull out the sonotube and use Borax to bring the pH back up. Hopefully it won't end up like the old woman that swallowed the fly.
Open to easier suggestions. Preferably a non-chemical more mechanical removal method. And yes, that idiot was me.