Rust removal for fuel tanks (any metal, actually): 2 techniques, with links.

[sgtslag]

The first set of links go to a chemical bath, commercially available.  The second, is a write-up on how to DIY, using a battery charger, and a chemical bath, to convert/remove the rust from the metal.  While I have not used either method, I thought I would offer them up, anyway.  I found them on another forum, and they seem worth sharing.  Cheers!

------------------------------------------------------------------------------
Commercial chemical product:  http://www.metalrescue.com/home.aspx

Videos on how to use the commercial chemical product:  http://www.youtube.com/watch?v=F6UjAn7qO7Y


************************************************************************
******* DIY method, with electrolysis(?), and a safe chemical bath you mix up yourself: *******

Supplies: rusty tank, medium sized steel or zinc plated bolt, piece of 10-14 gauge stranded copper wire, battery charger with 6 amp output (higher is faster), sodium carbonate (pool/spa chemical ph+), and water.

Remove the sender if the tank has one; if not, shake out any loose bits of rust and vent the gas vapors from the tank by leaving it open for at least a day.

Mix 1/4 cup of sodium carbonate solution for every two gallons of water. Stir it up well and pour the brew in the tank (you might want to make a little extra, as some usually spills).  [You need enough solution to completely fill the tank, plus extra, to replenish the tank as the process evaporates solution, over time...]

Clip the battery charger ground lead to the tank body in a clean area. Strip both ends of the stranded wire, one end 1/2-inch and the other end about 2 inches. Split the strands on the long end, and securely wrap the wire around the bolt. I use a 5/16-inch diameter bolt about 2 inches long, either steel or zinc plated.

Immerse the bolt in the solution and attach the wire lead to the positive terminal of the charger. Set the charger output on the 10-amp setting. The mix will start to bubble a bit, and rust and gunk will form around the bolt and upper tank opening. Rinse the bolt off every few hours, carefully pouring additional sodium carbonate/water solution into the tank opening to flush out the gunk there as well.  Keep the tank as full as possible.

After a day or two, as the process slows down, the amount of gunk generated decreases. Most of the bolt will disappear as well, so keep an eye on the copper wire holding it; the bolt might need to be replaced.

When it looks as if the process has finished, rinse the tank with clear water. Slosh it around and even use a handful of 1/4-inch nuts on a string to scrub out any loose residue. Rinse again and dry with warm air. A vacuum cleaner outlet hose or hair dryer can be used to supply drying air. If I'm not completely certain the tank is dry, I'll use a quart of denatured alcohol in the first tank of gas to remove any moisture that remains [I would use SeaFoam, instead:  it will absorb water, without adding alchohol; SeaFoam is 100% petroleum distillates, safe, and it will clean out minor varnish depsosits within the fuel system].

This process only eats up rust, not steel. It converts rust to black oxide, which is a more inert material. Unlike acid etching, the surface left behind is rust-resistant. Acid etched steel left bare is much more likely to re-rust. This process works well, the chemicals required are safe if properly handled, the tank shouldn't need to be coated afterwards (unless it's leaking), and the time required is minimal.
------------------------------------------------------------------------------

[shinyribs]

Electrolysis rust removal works great. I've done it several times with auto parts that had hard to get at areas. Riveted together hinges,for example,that can't be bought but also can be separated very easily. I never found that the solution evaporated all that much,but usually it would get a build up of scum on top that will bubble up and sometimes overflow the container I was using. When mixing the sodium bicarbonate(washing soda-nut baking soda works just as well IMO) I just stir in as much as the amount of water will absorb. Any excess will just settle to the bottom and won't hurt anything.  

I say all that to say this: IF you are gonna do the inside of a gas tank,and you like the paint on that tank, cover the outside of the tank to protect it from gunk overflow.

A layer of easy to remove painters masking tape under a good layer of duct tape is the preferred method from what I've read. If you use the duct tape directly on the tank you might not get it off with good results.

Something else I've seen others have good results with,but no personal experience myself,is using straight vinegar. I can't see how it would work,but I've seen a couple times where it has really cleaned out a tank inside. No electrolysis,just pour in straight vinegar and let soak. Rinse out after with a baking soda/water rinse to neutralize the vinegar.

[TheLastJoker]

Does this method require line-of-sight? That is, would the metal down the sides of the center divide that can't directly "see" the bolt still be cleaned? I definitely like this method as it leaves a rust resistant finish afterwards so I'll be giving it a shot.  Muriatic acid was previously used in my tank but it is once again rusted.

[zer213]

Ive read that line of sight is a big part of the process. Im going to give it a shot on my tank, not much rust in it but enough to try out some science. I figure the best piece of sacrificial steel I can use is some steel wool. Cheap, readily available and TONS of surface area (thats a huge part of the process).