The problem with ALL of the tub refinishing products is that they just can't match the hardness and durability of a powder coating. Klenck makes an epoxy bathtub paint, but even epoxy is not as hard and durable as you need in a bathtub that should be lasting you 40+ years.
So, if I were you, I would just replace the tub. American Standard makes the "Salem" tub which is heavier than a regular enameled steel bathtub because the steel used in the Salem is heavier gauge and the powder coating on it is thicker. It's the only tub that I know of that comes close to the kind of bathtubs that were made in the 50's and 60's. Nowadays all the other enameled steel tubs from American Standard and Crane are made from thin gauge steel and don't have as thick an enamel coating on them. And I find that so extremely annoying because of the amount of work you have to do to replace a bathtub. It's not like changing a light bulb. If you're going to go through all the work of cutting out the plaster wall around the tub, and then installing new wall and tiling (or whatever wall surface you choose), you're going to at least want to install a strong durable tub that will last you 50 years. But, apparantly, the tub companies just aren't making as durable bathtubs as they did years ago.
You simply won't find a field applied coating that will dry to as hard a film as you need to provide truly good service on a bathtub. Your two best options are to either replace the tub, or to plan on refinishing your bath tub every 5 to 10 years or so.
You don't need to know the rest:
The reason why powder coatings are so much more durable than field applied coatings like epoxy is because they use a completely different technology. In powder coating technology, a metal part is electrostatically sprayed with a mixture of hard plastic particles and coloured particles called pigments. The electrical charge on the plastic and pigment particles attracts them to the oppositely charged metal part. That part is then baked in a special oven for anything from a half hour to 2 hours at anything from 300 to 1200 degrees F. (depending on the kind of powder coating). During the baking process, the plastic particles melt and flow together to form a smooth layer of plastic over the metal with the coloured pigments suspended in it very much like raisins in raisin bread. As that powder coated part cools, the plastic hardens back up to it's original hardness, and that is typically about 3 times as hard as field applied coatings like epoxies and polyurethanes. It's that high hardness that results in the enamel on a steel bathtub still being smooth and glossy after 50 years of use.
Generally, the harder the plastic used to powder coat a part, the higher the baking temperature needed. The hardest coating in your house is probably the special form of powder coating (called a "ceramic" coating") which is the blue-grey coating on the inside of your oven. Those are baked on at about 1200 deg. F.