I used to repair gas tanks at the radiator shop I worked at.many radiator shops also repair gas tanks.they are boiled out with a caustic tanks (no boom boom) then the rust is ground away the area is then heated and cleaned with muratic acid and tinned and soldered like old school body work.depending on how bad the tank is most times the repaired area is much stronger than the rest of the tank.they should be able to tell you if its worth fixing .seen alot of time wasted by doit yourselfers trying to fix them