Myra, I have never tried this but it is the way I would proceed. Clean the tub thoroughly, sand to rough up the surface with about 100 grit paper, clean well again, buy Hobby Poxy paint from your friendly local hobby shop, spray paint outside on a dry day with as little wind as possible, after it is dry, sand and apply a second coat according to manufacturers directions. Hobby Poxy paint is a 2 part epoxy paint designed for model airplanes, boats, etc. This should give you a good durable surface for water exposure. By the way, you will need their thinner for spraying or for clean up. Let me know how this works.