Hi again: The product I have used is called Klenk's Epoxy Enamel Tub and Tile Finish, available at most HIC's, like Home Depot. Its a two part epoxy that is easy to work with.
As is always the case with doing a quality finish, 85 to 90% of the work is preparation. I have applied this finish to two tubs, both have come out great. Let me briefly go over how I did mine.
After taking off all hardware and digging out all caulk, I packed a small rag between the now open tub drain and the drain flange underneath it to make sure the rinse water I'm about to use goes down the drain and doesn't collect under the tub. Then I use a strong cleaning/degreasing agent like TSP to scrub the old enamel surface to get out all old body oil, etc. Be sure to wear gloves and ventillate the room well. I used a brillo pad to scrub with. After rinsing, use 220 grit wet and dry sand paper and sand out the old enamel with clear water. This will take quite some time. Repair chips with Klenk's (or another) epoxy chip repair kit. Be sure to sand it flat. Rinse. Next etch with acid. I used straight Hydrochloric acid (Muriatic) from a pool supply house, applying with an old synthetic sponge and rubber gloves. You don't need much. YOU MUST VENTILLATE WITH FANS AND OPEN WINDOW. Let set for 10-15 minutes and rinse well with water. After the surface dried, I use a final cleaner with a very clean applicator. My favorite is acryli-clean by Dirzler (normally used to clean an auto body just before painting....its available at auto paint stores), although laquer thinner will probably work too. The tub needs to be at least 65 degrees before applying enamel. Follow the directions on the cans. BE SURE TO USE A QUALITY CHINA BRISTLE BRUSH. Purdy makes a good 2 inch brush. It is a bit pricy, but is very important to getting a good, flat finish. Be sure to apply two coats, lightly sanding between coats with 400 grit wet paper, dry and wipe again with acryli-clean before applying last coat. You should get a beautiful and lasting finish.
The only small problem I have noticed two years after applying this finish (white) to my current tub is that it has yellowed slightly. I have not used a strong cleaner yet (I'm a little afraid to), although Klenk's tells me this is normal and occasional cleaning with a strong (but non-abrasive) cleaner will not hurt the finsih (they have a 1-800 number on the paint box).
Anyway, this is how I have done it. Hope this provides a few tips for you. Best of luck.