If none of those things work, use a scotch-brite pad. We use the rust colored ones, but they are the most abrasive. Start rubbing lightly, with water, only on the rust spots, then rub harder as necessary. After removing the stain, if it has dulled or scratched that spot, use buffing compound (again, only on the spot). Rub it in, circular motion, then wipe it clean. Should be like new. If the tub is fiberglass or acrylic finish, be very careful, as the dullness and scratches will show much more than on porcelain. (Refinishers secrets)