The more I see of mig weld body repairs on old metal on TV, the more anguish I feel for the folks that buy the restored vehicle for several thousand ££££'s. In my experience MIG will only work well on new clean metal. Plug welding a new panel onto a joggled edge still carries the risk of distortion on unsupported original panels....a rear wheel arch for instance. Try and run even a short bead and the buckles still appear. So all these restorers use a join the dots method, hardly a recipe for a good strong cohesive fusion joint.
Brazing is better for a tank repair. Like soldering, the parent metal is not melted, but the filler material added at a lower temperature so less chance of distortion. A brazed joint is much stronger and durable than a soldered one, but as always distortion is your enemy, and success depends to a degree on the rigidity of the metal surrounding the repair. Soldering or brazing needs the parent metal to be clean, Bakers Fluid was the acid dip in a can, back in the day. You bought a can, did the job, and of course the next time it was needed the contents had turned brown or the stuff had eaten thro' the can. Nice to see it's still available, now packed in a modern single use plastic bottle.......Progress indeed.
Swarfy