look up POR 15 paint over rust, they have a repair for old rusty leaking gas tanks. First pour in an acid to remove rust and scale, then after that is emptied a neutralizer goes in. Finally a 2 part epoxy mix is put in and rotated around the tank. it is poured out before it hardens and allowed to dry. My old Ford Jubilee replacement tank was over $300 this fixed it over 10 years ago still no leaks!