I have an old rare model Honda Gyro scooter I am semi-restoring (replacing the trashed engine and other bad parts) and the fuel tank has a MASSIVE amount of thick, black varnish in the bottom. When shaking it out, it looked like clumps of mud. (And it could be mud, given the transmission had mud in it.. but it's varnished too) I removed as much of that as I could, then took the garden hose to it until water was coming out fairly clean. It only has one accessible hole down into the tank, and that's the couple inch wide filler hole, so I can't reach my hand down in to clean it out. When I scraped the bottom with a flat head screwdriver, thick black crap came off (like what was shaken out) and I could see the shiny metal of the bottom of the tank, so it's not rusted out in the bottom. What's the mildest liquid solution that might help dissolve the varnish without being able to reach in and scrub it? If it has to sit for a week, that's fine. Some people like to use metal nuts to bang around, but I'd be concerned about sparks. Some people say to use gravel, but that can shed particles. Glass marbles or instead maybe?