If it was rusty I'd recommend an acid cleaning but since it's mostly varnish if it was me I'd start with a gallon of cheap mineral spirits paint thinner. Pour it in and slosh it around then let it sit for a while. See what comes out. If it looks like it's doing the job, repeat using fresh thinner until you're satisfied with the results and charge on. If the mineral spirits doesn't cut it try lacquer thinner. I know that will cut old gas but it's expensive. If you're not showing rust you most likely won't need to coat the inside of the tank with a liner.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.