If the porcelain on your bathtub is not chipped but merely discolored, you can probably get rid of the discoloration with a commercial cleaner. If the stain is stubborn, get some oxalic acid from your drugstore. If, however, the porcelain is chipped, there is no paint available that will do the job properly, as it is almost impossible to successfully apply paint to the surface of a bathtub. It will look good for several months, but after that the paint will begin to chip and decay. Professionals can refinish the bathtub with special coatings, but they are expensive. If your bathtub is an old tub that sits on four legs, it can be removed by a professional and taken to their shop. There they will grind off the old enamel and apply a new coating. If your bathtub is attached into the wall, then removing it will create more problems than it is worth.