The "reddish color" is the original fuel tank "liner" material. Guessing it would require submersion in a chemical vat to cook out all the interior chemical coatings. Try a radiator shop.
You'd end up with a bare metal tank. Then add the POR-15 (also used in aircraft) to properly seal pin hole leaks. Might need to build a rotisserie fixture for sealer application. However, the vatting might expose extensive damage that may be beyond the POR treatment capabilities. Then there's the exterior primer and refinish work.
Just hinting here that you may want to be prepared for the worst while hoping for the best. New tank, used tank, fix existing tank... as the sign sez, "Welcome to Las Vegas."
Good luck on whichever trail you take. Monte