It is quite possible. They also don't last, so spend the extra money and get a porcelain covered cast iron tub.
For more information:
Read "What's less toxic: refinishing an old cast-iron tub or getting a fiberglass tub?" a Q&A answered by Molly McCabe.
Also, read Mick Dalrymple's Q&A "I have a bathtub that needs re-enameling. Is there a nontoxic way to do this?"