POR 15 in a dunk tank. Company local here is using this product in a dunk tank to reline both the inside and outside of the fuel tank.
Worked a charm on my C52a Colt, as it had leaking seams and pinholes up on top. The stuff actually gets harder with contact to moisture.
I sold that gas tank with the car and it outlasted the whole car with no issues from what I was told years later.
You could always paint the tank yourself and even clean and epoxy first, as mentioned above.