I wish you luck, but if you can remove varnish from fuel tanks, without removing them, with detergents, degreaser, or solvents, you’ll be my new hero. I didn’t try alcohol, but I did try lacquer thinner and acetone. They removed the soluble constituents, but left a crust behind. I put acetone, lacquer thinner and marbles in my tanks and carried them around in the back of my truck for a few weeks. The marbles knocked loose some of the solids. After siphoning out the solvent and cleaning with detergent, this is what came out:
Ultimately, I had to invent the Tank Tumbler TM to get the last of the varnish.
Like I said, I wish you luck, but I think you’re going to have to pull the tanks to get the varnish out.
Dave
P.S. That was a nice post about the family history behind your boat. You done good.