Yeah that pic looked like a FCAW (fluxcore) job after a few beers, at least my FCAW welding after a few beers.... Likely need to be sealed...
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I have welded one or two tanks before when I was a no money miracle, I totally get why a shop would say no.
If silver solder (brazing) will do the job that may open up some DIY without having the real welder. I use it on Bicycle frames in place of regular brazing. Just make sure it is real silver solder and not plumbing or electrical silver colour, it is not cheap as it has lots of silver in it.
