My tank was super rusty, with rusty fuel pouring out of my under hood filter when I pulled the hose off. So, Pulled the tank. Local radiator shop (the only one that would hot tank fuel tanks wanted $485 to clean and seal with over a month turn around time). So, I set out to clean it myself. Tried a few methods and here are my results: shook out 10 pounds of dirt consistency rusty crap and rust flakes out of it. Used electrolysis for 2 days, changing out the sacrificial steel and soda solution several times. Some, but not great results. Next, swished about a gallon of 1:4 muriatic acid:water a few times, then let it sit over night full at same ratio. It was looking pretty good now. Rinsed with water then dumped Ospho in it, swished it all over, drained it then dried it with a heat gun. I figured the Ospho had converted the rust to iron phosphate. Last thing was pouring a quart of ATF in it, swishing then draining (see pic). See the fuel return tube in bottom right of pic. That thing was clogged solid. I tried forcing wire through it, air pressure, to no avail. I stuck my propane torch in the tank, heated the crap out of it until it was smoking then hit it with air, then.... Thupe!!, the clog came out in a wad. I filled the tank with fuel and plan on keeping it full to prevent rust. I decided against sealing. We'll see.

Bottom line. Muriatic acid worked the best at cleaning and Ospho may save it from rusting again. 

Good work, I'd be interested to see if the rust comes back especially on the roof of the tank.  Muriatic acid is very effective at removing the rust but I've noticed that it resists being neutralized almost like it gets into the metal.

If it got that bad once, it will do it again.... I would coat it.

7 hours ago, grannyknot said:

Good work, I'd be interested to see if the rust comes back especially on the roof of the tank.  Muriatic acid is very effective at removing the rust but I've noticed that it resists being neutralized almost like it gets into the metal.

I neutralize the muriatic acid with a strong base.

Any industrial strength greasecutter is great for this.