I'm mates with a local stock feeder and they buy molasses in the 1000 litre pods and decant it. To decant it they turn the tap on and let the molasses run into a piece of guttering with a few holes in it that they place the bottles under. Apparently the guttering is eaten away surprisingly fast.
For a gas tank I'd try reversing the electrolysis procedure as mentioned at the bottom of the article for cannon barrels. Perhaps use the phosphoric acid Gadge spoke of and swill it around.
