Sheet metal workers solder steel with a big iron, flux and a stick of 60/40 solder. 50/50 is what body guys use.
If you use a torch, I wash the tank with alcohol to remove the gas then water to dilute and remove the alcohol, leave the cap open or masking tape it just in case. Fill it with argon, nitrogen, co2, any inert gas will help. Dry ice works too, it's co2.