Just bought a 1952 Pontiac. Still have the straight 8 in it. I pulled the gas tank. Been sitting last 10 years. I have never seen such a mess. Inside the tank was a 1/4" thick varnish/old sealer jelly like substance. I was able to tear off a few sheets of it and look at it in the light. Hard to tear apart, mean looking crud. Lacquer thinner, gasoline barely touched it. I went to the hardware store and found a sulfuric acid based drain cleaner. Says " Danger for professional use only" and inside a quart jug and a plastic bag. Common stuff. Came back and dumped it in. Started a reaction right away. Making a rotten egg fog that seeped from the tank. I could feel the heat as it chemically reacted with the tough jelly on bottom of the tank. Let it sit overnight and before work, with eye protection, dumped in a quart of acetone. It was still hot. Came home from work, and it had worked. Totally dissolved the jelly and I poured out a black substance like melted tar. Pressure washed tank, and presto, 99% clean to the shiny metal floor of the tank. Now I can deal with the last bit. Just thought I'd share. Man that stuff worked great. Now I just have to find out where #1 tdc is on these straight 8s so I can time it. Blasted around the yard pretty good right now with a temporary tank in place. Need to free up the valve body on the '52 hydro.