The hardest and most important part of any painting is in the prep work as well as the quality of product used to refinish. Tubs are finished with a very hard surface from the factory due to the fact it is applied as a powder coating and then baked at very high temps making it very impervious to water etc. That slick hard surface is very hard to prep for refinishing. Not only should it be chemically etchrd (acid) but manully sanded to give the surface a tooth for the paint to grab on to. THis is very important to asssur any finish will adhere to the tub surface. I'm sure the body repairman is doing this in the shop, He will first wash it down with a solvent to remove grease etc, sand it with various grits , wash it again, chemically etch it , spot prime with an epoxy primer and then spray it with at least two coats of a two part epoxy paint, and maybe bake it. If this process is followed it will give a very durable coating that will last for years. Very difficult for a regular homeowner to do all of this, but no matter what (even with the home center epoxy systems) cleaning , mechanical sanding, and chemical etching wil provide the best surface for good adhesion. If the finish is coming off, it is do to poor adhesion to the surface underneath and the expansion and contraction of the paint on the surface (due to the cool surface being hit with hot water is causing the film to lift and peel.