I have a '78 750F and some PO had lined the tank with some kind of "red" epoxy material. It was turning the gas red which was also confusing, but now makes sense - the ethanol was attacking the liner. Another issue was the liner had been put over rust, so I not only had to remove the liner, but than treat the rust. WARNING - never use a tank sealer to cover up rust. If there is a good part to my story, the outside was in need of repainting, so I didn't need to worry about being careful with the existing paint.
I read everything I could find on the internet and decided on acetone. It worked, but took multiple applications and 2 gallons of acetone at $20 a gallon, and then I had to find a place to discard it when I was finished (and that is not part of my story).
I treated the rust with baking soda, water and battery charger. Again, this process took two applications, and the tank still doesn't meet my expectations. I've used this process on a number (more than 3) of motorcycle gas tanks and they all came out like new metal inside.
I'm running this tank on the bike (but the bike is still being restored and not on the road yet). I'm also fighting carburetor issues and believe they are related to the condition of the tank before being cleaned.
Hope this is helpful.
Later, Bud...