Yes, it is tinned steel as far as I know, also using copper tubing, flushed the tank with fuel to see if if had trash in it, then I stored it, when I got it down, I pressure checked it, then found the leaks, no they weren't in the soldered seams.
All the leaks were either in the bends, or the flat sections.
I used non corrosive flux, and rosin core solder that I have used many times before with good results...
After repairing a couple leaks, I pressure checked it again, and it showed leaks in the flats AGAIN!! That get's frustrating to say the least.
I've built tanks before, but not with this metal, will go to tin can stock on the next ones..
Sure do thank you fellow's for your help/suggestions