You might check this thread out, some discussion on the topic:
http://www.airforums.com/forum...=bathtub+paint
My hubby repainted our fiberglass "bathtub" in our 63 Bambi and he used a spray can of the Krylon Plastic paint available at about any hardware store. It hasn't been too long yet (a few months) so I cannot attest to how it will hold up. However, so far, so good. He said I shouldn't use cleanser or anything on it, just mild soapy water and a rinse.
Good luck! You can also use the search box above on the blue toolbar and key in bathtub paint ~ you will get quite a few hits.
Mrs. NorCal Bambi (traveling in S Tardis)