I know Eastwood sells a tank coating kit. Cleaner and a sealer to stop any rust. I think it's Eastwood.com. I haven't used it but a local radiator place offered to seal my tank so I imagine its the same stuff.
there you have it. nothing better than experience be told. I would have used it but I bought a stainless tank. Maybe someday I will take the old tank to have it cleaned out. then I'll use that eastwood stuff.
Eastwood is to coatings & paint supplies as Speedway, Summit & Jegs are to parts. High quality products that work..... great customer service. Cheaper stuff available elsewhere .... but when it comes to things needed for a quality build "you pay for what you get".