I'm far from a Profi in the field but I have looked into it before. There are companies that will do it either in home or take it to them, but often it is expensive. There are many home repair kits available, but the paint on / fingernail polish kinds work poorly. The sand the whole tub down and put a new layer on are better... But the cost and work typically don't justify it. If they are just top surface damages, you could try a higher polish sand job on the tub and see.