This happened to me when I was moving out of an old apartment building- the night before the inspection! I had used toilet bowl cleaner in the old tub, and the rust had leached through the worn-thin enamel. I called Johnson Wax, the manufacturer of the bowl cleaner and they told me to use Rit (dye) Rust Remover- which worked. I think I found it in a drug store in a small bottle and had to buy several to do the job. Good luck!