| You didn't say what type of tub you have? If its cast iron or enameled steel it can be resurfaced, but as was mentioned by Handymac, it will require a professional service, it is not a DIY project. If it is fiberglass you can use a power buffer and the fiberglass rubbing compound that they use to refinish boats and automobiles. If its acrylic I haven't got a clue how to resurface or repair it. | 
