as a bath tub repair and refinisher, go to the yellow pages and look under bath tub repair and refinishing. You just cant use anything, it will not work/last. Professionals have bath tub coatings that come strictly from a company that deals with bath tub refinishing. They should grind the rust off then fill it with filler, sand it to the shape of the tub, then primer/seal it, then coat it. They may have to tint a color to match the color of your tub.