I was just about to give up and replace the tile (and I had already bought an aftermarket enamel paint kit on the tub) when I accidently found out that whatever the "TSP" stuff is nowadays (without phosphate) seemed to help- just a little. Now, if I spray a tile, then use a comet-type cleaner along with a motorized scrubber, I can get one tile clean about every 10 minutes. So....
now I know that this stuff comes off, and that the tile looks ok underneath. The tub looks ok underneath too, in my test cleaning areas. But my wife is gonna kill me if it takes me 2 years to clean this tub, with nothing else getting done in the meantime. What else should I be trying to get this residue off? There has to be something strong enough (and relatively safe) that I can use to make this go faster. I'm already planning on bleaching and re-sealing the grout, so I don't mind if whatever I use eats off the old grout sealant, assuming that any is even still there (besides, that would just make it easier to bleach the grout).