I oiled the crap out of my tank after cleaning with muriatic acid. It sat with a few liters of oil for a couple months, and it had rusted right back up. Maybe it was the muriatic acid, but I'm thinking if I've done a good job of getting everything out this time with metal rescue, and I'm betting it'll rust again.
I'd bet they used galvanized steel when building the tanks. Now 35-45 years later, the zinc has corroded away, leaving the steel, which has rusted, and after we remove the rust, all that's left is steel, vulnerable to more rust.
That said, fine. I'll try it. I have the caswell kit here, and it cost me $100 (Canada), but I'll try without. I'm draining the metal rescue tonight, using dry coat to prevent rust while the tank is painted, and I'll double check there's no rust before using it when I get it back. I'll save the metal rescue, and I guess use it again next year if I have to (after which I'll line it)