If you are only dealing with old gas and varnish from it and rust is not a problem lacquer will clean it. I just did one and it came out great.
I got 5 gal of cheap lacquer thinner from NAPA (a 5gal can was about the same $$ as 2-3 one galon cans). I poured the thinner in the tank and let it set for a day. Turned to the other side and another day. Four sides four days, slosh it around a little once or twicw a day. You have to plug all the holes with something the thinner won't affect. When done flush and wash out with a water hose and dry.
When done the thinner went back in the can and will be reused for the same thing again.
