Clean/derust the tank and if it does not leak DO NOT coat it. If it does leak find good used tank and toss you tank in the recycle bin. To me lining a tank is a last-ditch effort if the tank leaks and you cannot find a suitable replacement. Lining a tank is a process that needs to be followed to tee. Even then it's likely you'll eff it up some portion of it and the liner will eventually bubble and peel and cause more trouble than just cleaning the rust.
Cheers,
P-14
Cheers,
P-14