The local Radiator/Gas tank rebuilder here uses POR15 grey to seal pinholed and repaired tanks. I had pinholes and or a possible seam issue on a Colt I owned.
Sent in the tank with pinholes (where the rubber rust undercoating could not be sprayed with a tank installed), and received back a very shiny, sealed and oh-so grey
fuel tank in return; my old tank.
So, I highly suggest the POR15 route is you can first figure out how to soda blast or clean the area in question, and then you can dip/roll/spray the tank once it is rust prepped.
I have personally used POR15 stuff before, and it is rock hard and works well. My tank was literally sealed in a thick shiny grey layer. I really liked it, and the tank
most likely outlasted the car with it's final owner, Colt = RIP now.
Edit - Just found this:
click