[blucar]

I could not begin to tell you how many gas tanks i have repaired and/or built during the past sixty plus years. In the early days of RV'ing I built many AUX fuel tanks for my RV and several friends.
In the mid 1960's when I was building fuel tanks wire-feed welders were not very common, electric welders were not much good on light steel so I used my Victor OXY Acetylene torch that i used fro body repair work.
I would have the local steel supply shop shear and bend the 16 gauge to my specified size then I wold gas weld the remaining seams Once the tank was finished I would hook up a tire pump to the tank, filling it with with a few ponds of air and spay liquid soap/Windex on the welded seams to check for leaks.
Having lived in rural Idaho in my youth and working in several repair shops, repairing rock damaged fuel tanks was almost an every day affair.
We would drain the fuel, remove the tank, flush it with water, then wave a gas torch over the open fuel filler neck, if there was any fumes left in the tank they would burn off with a light quick burn-off of the fumes.
Soldering the tank with an Oxy torch was very simple once the metal was cleaned and 'tinned'.. I said Oxy torch because propane torch's had not been invented yet in the 1950'-60's. The closest thing to a propane torch was a white gas blow torch and a soldering iron that was heated by the blow torch. Hmmmmm, still have one of those in my garage collecting dust.
Actually, IVORY Soap is a very good semi perminent gas tank repair.