At the bottom of cat litter boxes is usually a dark brown or gray mineral that doesn't scrub off. A few years ago I tried using CLR I think it was. It bubbled up and got rid of it. It did not hurt the plastic litter box at all. It worked amazingly well. But the strange thing is that when I tried it again a couple of years later, it didn't work at all. I don't know why it would work one time then not again. I don't know whether to invest in another bottle of CLR. I don't have any other use for it. When I rinse the litter box under the tap running full blast it causes bits of the residue to flake off, but only a small percentage.
Does anyone have any ideas?