[sixTsix]

downlow , my nephew recently purchased a 71 c10 with same, 20 yr old gas in it. This tank was baaad man. So bad it smelled of varnish and had very large chunks of what I believe was left over lead and the other things that come in gas, lol! We tried several methods that we researched. The end result was we got it clean and flushed out. The first method we tried was running some flush thru it. that was a no go. Now im not sure of which flush he bought, I wasn't present at the time but the results weren't good enough. This tank was pretty bad, large chunks in the bottom as well as gunk on the side walls. Second we tried running some denatured alcohol thru it and let is sit a bit as well as sloshing it around ..flushed ...still not good enough. Third we tried electrolosis. This method work ok, although time consuming, we still weren't satisfied. Finally we poured 4 or 5 gals of cider vinegar and let soak in the tank for 2 days, maybe even 3. Every day we would slosh around and lay the tank on its side to break down side wall gunk.
I have to say the vinegar method worked the best for breaking down the side wall gunk and rust as well as the bottom. Once the 2 or 3 days were up ...we then flushed with soap and water. The large chunks were still present but all the side wall crap was handled very nicely. For the large chunks we took a shop vac with a coat hanger strapped to the hose (wire coat hanger for breaking down big chunks and scraping the bottom) ran it in the sender hole and suck all the rest of the garbage out. Once completed, then flushed with soap and water again and let dry out. Tank is very well usable and looks hellaov a lot better inside. For the price of the new tank versus taking to a radiator shop to have boiled out, these methods were the cheapest and the best part about it , didn't have to rely on anyone else to get the same results
hope this helps.