Plain old mild steel. The last tank I built has been in use for 5 years now and as far as I know there have been no issues. The one before that has been around over 15 years without issues. Sitting around not being used for years, 1/2 full, is what promotes rust and other problems.

I would be a little concerned about coating the inside of a tank these days, this stuff they are selling us they call fuel has proven to cause reactions with many of the tank sealers from even a few years ago. I don't think they are done messing with the fuel yet, I expect it to get worse. As long as the new car manufactures keep making tanks from steel, (though they are galvanizing them) I think we will would be wise to do the same. Gene