I do like phosphoric acid and I also like the "Calcium, Lime, Rust CLR" bathroom cleanser. Getting anything mentioned here and above hot . . . boiling even . . . seems to help in getting rust and varnish out of the tank. The heat softens up the stains and rust and seems to generally rev up the chemistry. I use an old camping stove to get it hot and then agitate the tank with a hundred or more nuts, bolts, washers, screws, railroad spikes and old sledgehammer heads to break up the rust. Don't forget to turn the tank upside down to clean the top surface - even though you can't see it, it'll be rusty and stained, too. Just kidding about the spikes and hammer heads but you get the picture. And boil whatever fluid you use outside because it can stink up the house pretty bad.
Whatever method you chose here . . . . DON'T BE IN A HURRY. This could take a week or more of active effort on your part to get it all cleaned out down to bare metal. A good effort here in getting it all cleaned out will pay dividends in the next steps of discovering any pin holes in the metal and when you finally put in whatever liner you chose to prevent future rust.
The fuel tank on my current CB500 resurrection took a week to get clean and I really thought it would be pin holed but it wasn't. And then the urethane liner went in from a motorcycle-sized "POR 15" kit (in silver urethane, not black) and it has worked out great. This was my seventh POR 15 tank repair and I have never been disappointed but preparation is the key for any tank liner you chose.
Rick
