Their is no way to properly repair a cast iron tub. The finish is baked on at 800 degrees or so and can only be applied once. What you are doing is just painting the tub. Painting it with the tub repair stuff is good for a few years. Their are professional refinishing companies that you can have come and paint your tub for you. These finishes are much tougher than then ones available to your average consumer. They only have a lifespan of about fifteen years though.
One thing I can tell you not to do is have a handyman come fix your tub. He'll just use the same kit you have, you need to find a good refinishing company.