POR15 is only one way to only one way to fix that old gas tank. I did as the instructions said, cleaned the inside with the proportion of Muriatic Acid water suggested. (it etches the interior, protect your eyes etc.!) I then duct taped the openings on the outside with our friend, Duct tape.! It held the epoxy coating while it was applied.
Two things, be sure to remove your sending unit of course! The other being, when I did it, the "Liner" material was available in 2 grades; Alcohol & non-Alcohol.
Be sure to check it is capable & safe to use with our modern fuels! The non alcohol is/was considerably cheaper! I’m told it will dissolve, & really really gum up your fuel system!
Even if your tank isn’t leaking, you may already have it out of the vehicle, I’d suggest doing it. Our vehicles for the most part sit for long periods. Condensation may well do in a reasonably sound looking tank later on. It would be a shame to mess up a redone underside to remove it later. Its not that bad a chore as long as its out already.