I had accumulated quite a stack of tubs that needed cleaning out and disinfecting, so I filled my little-used porcelain bathtub 2/3 full of cool water and dumped in about 1 1/2 gallons of bleach. Over the next several days I rotated my tubs through, so that they each were submerged in the bleach solution at least 8 hours. When I pulled out the last batch, I found that the bleach had gotten through invisible cracks in the porcelain and reacted with the cast iron, causing large ugly rust stains over much of bottom of the bathtub . Normally I would use bleach to try to get something like that off .
To replace the tub would entail replacing all the tile in the bathroom because tile is installed over a lip on the bathtub that runs a short distance up the wall. To remove the bathtub you have to chip out the tile. The whole bathroom is tile and they don't make that kind of tile anymore...my house is 60 years old. Guess my only option is to have the tub reglazed, but they never look quite right after that. Anyway, hope I can save someone else some grief by my experience!