A good external epoxy is a good idea. Quite a few of the production steel gas tanks were left unpainted, and some rusted clear through from the outside when the foam under the hold-down strap collected and held water against the tank top & sides.
Whatever you use inside, given the trouble we have in pulling tanks, be very sure the tank is perfectly clean and the coating is absolutely guaranteed for use with alcohols in our gas (in ever-increasing %). Some motorcycle sloshing compounds were/are attacked by methanol or ethanol, come off and clog fuel pumps and carbs or injectors. Not too simple to correct on a bike; even worse on a Pantera.
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