I have no experience here, but a guy I used to work with had redone his and his Grandmothers. The product he had used was called "Klenks Epoxy paint". The stuff is made specificly for bathtubs.
One of the other guy's here may have expirence in this. I'm certain that there are tricks to this stuff to get professional results. If your a perfectionist, like I am, then hire a pro, they know the tricks.