I have finally stumbled on to a way to de-rust a gas tank. I had a 79 DOHC tank that was completely covered with rust inside. While searching the net I came across a website called Theruststore. They have several products for rust removal. I chose Evapo-Rust. I emailed them to see what they thought about the application I intended to do. Got a reply from Rusty! Yeah Rusty. He said they have never had the product used in that way, but it should work. The stuff is not that expensive. $24.00 a gallon. I decided to go for the 5 gallon size which came to $99.00 delivered to my shop. First I poured about a pint of Castrol purple cleaner with a gallon of hot water in the tank and swirled it around for 10 minuets. Then dumped it out and rinsed with 2 gallons of hot water. Poured that out, reinstalled the petcock, and filled the tank with Evapo-Rust. Instructions indicate a soak of about 20 minuets should be sufficient for light rust. I had more than light rust so I decided to let it soak for 24 hours. The next day when i looked inside, I couldn't believe it. Nice and shiny, like new again. The solution was not discolored any more than when it went into the tank. So I put a hose on the petcock and ran it back into the pail it came in. Then I rinsed with hot water a couple of times. To get the rest of the water out, I took a piece of stiff wire and crimped a paper towel balled up on the end. Place that through the fill hole and dabbed up what I could. Next came the hair dryer. Blew that through the filler hole with the petcock off the tank for about an hour. The tank was good and hot after that! Seems to have worked well. Then I put about a cup of Marvel Mystery Oil in the tank and rolled it around to get a good coating inside. A couple of days later the tank was still looking brand new inside. I had sold the bike to a woman and she could not pick it up for a while. She was amazed at the results, as she had seen it before the treatment. The best part about Evapo-Rust is that it can be used over and over until it loses it effectiveness. I am still using the original 5 gallons on rusty chrome shock springs, exhaust collars and even some rusty tools.
Ed