Farmall H gas tank issues

Auto Body Shop Suppliers sell various acids to
eat, etch, convert rust. Plug fuel line fitting
before use. Some bikers put small size chain in tank, fill tank with pebbles and sand and shake and pull chain to dislodge big flakes

Paint stripper removes * anything * [ even paint ] Redi Strip = If a dealer is in your area
they can treat your tank [ process does not use Acid, but rather an Alkili and electric current
to gently plate rust off [ expensive ]

Treated tanks should be sealed, or they'll rust and leak. Check bottom edges of tank seams for pinhole bubbles [ tank rotted = junk ! ]
under paint. If the tank is wicked rare, check Hemmings Motor News, their is a company that lines old junky tanks with poly plastic [ very expensive ]

If you want to coat the innards, I'd use one of the comercial products like por17 or kreem.. etc.

soundguy