I was told today that you can cut out the rusted section of a porcelain coated cast iron tub and make the repair with fiberglass. This sounds a little fishy to me!
There isn't a good way to fix this in place. Some claim good results with liners, but I have no personal experience, only anecdotal reports (most not great). Painted on coatings are not likely to last anywhere near as long a s baked on porcelain on say cast iron.
Prep is the key, but only goes so far. A tub is a somewhat hostile environment - weight, friction, all sorts of chemicals from cleaners of the tub, and people, heat, not counting potential abrasives used to clean things up.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013