Hi, Mark. To answer your question, you have to understand 2 concepts. The first is that there are 2 kinds of epoxy. One kind is the true epoxy and the other is polyester. The true epoxy is like 5 minute epoxy, which has little odor. The other is like Bondo, which is a polyester and has plenty of odor. They don't like each other.
Tubs, surrounds and similar are made like a fiberglass boat. In a female mold, a jel coat is applied, usually by spraying. This becomes a thin surface on the outside of the finished part and contains the color. Stiffening layers of cloth, mat or shreded fibers from a chopper gun are then added to obtain the necessary thickness for strength. Polyester epoxies are normally used for parts like this since they are considerably cheaper than the true epoxies.
So the answer becomes clear. You have to find and apply a polyester based jel coat to the outside surface to disguise the damage. There are specialty stores that sell this product and that usually cater to industrial customers and boat yards. Check the phone book in your local area and, if unsuccessful, check the web for sources. I know that there are several. call and talk to them. Explain your problem and they should be able to give you direction. Color matching will be a big problem.
If this seems too much for you, check for suppliers of fiberglass tub liners for cast iron tubs. They may be able to suggest someone local that could help you on this.
Unfortunately, no matter what you do, the repair will probably not be perfect, but you should be able to do it good enough to be acceptable.