Here's a non abrasive technique that I've used on tanks in the past that has worked well for me. Get 2 or 3 gallons of liquid paint stripper, NOT the paste kind. Pour the stripper into the tank and slosh it around quite a bit. Pour it into a 5 gallon bucket and filter it through cheese cloth, then pour it back into the tank and repeat the process. Rinse with mineral spirits when you think it is clean. Some years ago, I put a goose neck camera into a 1952 VW tank that I cleaned that way and it was shiny on the inside, for the most part. Good luck.
