I've restored four rusty fuel tanks, to date, and have honed a process which works quite well:
- 1. Drain the fuel tank and make sure all traces of gasoline (petrol) are out of it.
2. Remove the petcock and fuel cap.
3. Seal the petcock hole so the tank will hold liquid. I've found that a small piece of wood (about the thickness of a ruler) works well, held in place with the petcock screws.
4. Go to Wal Mart and buy a gallon of Castrol's Super Clean degreaser. Pour half of that gallon into the empty tank and let it sit for about an hour. This will clean out any gunked fuel and will allow the acid to dissolve the rust.
5. Rinse the tank and verify that any/all gunk is out of it. If there are still signs of stale fuel, pour in the second half of the Super Clean degreaser and repeat.
6. Rinse the tank again and ensure that all of the degreaser (and it's foam) are removed.
7. Go to Home Depot and buy a gallon of Phosphoric Acid. It's green and sells as metal Etch and Prep. You'll find it in their paint department. The phosphoric acid will remove any remaining fuel varnish and will also remove the rust inside the tank.
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8. Pour half of the gallon of Etch and Prep in the tank and seal the filler hole. Duct tape will work but I have had the best luck with rubber bilge plugs. You can get them at your local Ace Hardware or in the marine section at WalMart.
9. Shake the acid around and let it sit. Rotate the tank slowly over the next hour, letting the acid sit on different parts of the tank. The idea is to coat every square inch of the inside of the tank with acid. You want to do this until you get all the rust out of all of the tank sections.
10. Dump out the acid and fill then tank with about a gallon of hot water.
11. Shake the tank thoroughly to remove the acid/neutralize it.
12. Pour out the water and pour in a 1/2 gallon or so of Denatured Alcohol. The alcohol will absorb the water and prevent flash rust. Denatured alcohol is very vapor potent so do this where you have plenty of fresh air and are away from any flame sources (pilot lights, etc).
13. Shake the alcohol around thoroughly and pour it out.
14. Immediately spray a can of WD-40 into the tank. Open up the petcock hole so the WD-40 can drain out as you do this. The WD-40 will carry out any remaining water/alcohol and will prevent the tank from rusting.
15. Finally, pour in about a 1/4 gallon of gasoline and shake around on the inside of the tank to remove the WD-40.