$500 gas tank?
That is what they cost new at a minimum. I've learned that screwing around with a 16 year old used gas tank purchased over the internet is not worth it. Odds are that they are as used up as the one you already have. And by the time you are done welding, coating and painting, you are into it for several hours/days, $$$, and have something that is not as good as a new tank.