We have a black stain on the bottom of a procelain bathtub. We thought that this stain was mildew but our old tried and true solution to mildew in the bathroom (a solution of 20% liquid laundry bleach that will eliminate mildew stains from tile grout, tub caulk, etc.) didn't even faze it. Scrubbing doesn't affect it. The stain doesn't cover the entire bottom of the tub evenly, rather it's somewhat mottled in appearance. It's a little rough to the touch. We live in Springfield, MO and have seen a tub in another home with this same type of stain and those folks couldn't get it off either.
Does anyone have any idea how it can be removed.