There are companies out there that can fix the rust stain, chips, etc on both fiberglass and enamel tubs, sinks, etc. In the Milwaukee area I use a company called Surface Specialists. Dont know who would to use in the area you live but a look through your phone book for Bathtub repairs and or resurfacing companies will help you find someone in your area. If you feel ambitious and want to try the job yourself you can order the kits from bathwizard.com
P.S. They are not cheap! One nick in a tub that I created doing a tile job cost me $125.00 to repair.