Best way to clean inside of a gas tank?

I found through trial and error that a few handfuls steel nuts or
screws, or both, combined with phosphoric acid worked the best for me. I dumped a 1/2 gallon of p.a. along with the nuts etc., plugged the fill and petcock holes and shook and rotated the tank for a 1/2 hour.
let it sit for a while and shook it again. I repeated that for about two hours then drained the p.a. ( using a large funnel ) back into the plastic jug that it came in. I then rinsed it out with 3 changes of clear water, shaking continuously and drained it. All the nuts etc. were easy to remove with a magnet on a flex shaft. The tank was then
clear enough of rust to use the Caswells tank liner ( after drying
the inside with a dry heat gun). I would never assume that a 35 +
year old tank has no pinholes, so I always use Caswells as a last step before prime and paint. Also ( very important ) be sure to use a
2k clear coat like Spray max gloss to protect against gas which will
bubble the paint.

 

I second this! Worked like a charm. I pulled the tank off and covered all the holes, including the gas cap, with Gorilla Tape. It held up great against the Cider vinegar. I filled it up with the full 13L and left almost no air gap. I propped the low end of the tank up with some lumber. I let it stand 48 hours. Then dumped onto the rocks beside my house. Magic!

 

I started out using vinegar and soon switched over to a product made for rust removal. The products that are made for removing rust really work well and I think do a better job. Remember to put something in the tank after you rinse it to keep the flash rust from coming back.

If you are going to do a job to it right and I think vinegar is not the choice to make for getting rust out of a tank. I started out that way too, so just trying to save you a step.

 

I found acetone was good at absorbing the last remaining water.

 

runs from 11 to 29$