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Phosphoric acid is also known as Os-Pho (tradename), correct? It leaves a white residue and can damage paint...
I have used Klean-Strip "Prep & Etch" phosphoric acid several times now for cleaning out rusty gas tanks (readily available at Home Despot for like $8 or something). It removes the rust well and also leaves a zinc-phosphate coating that helps prevent flash rusting. Once done, I neutralized the solution (you mix it with some water) with baking soda and water then rinsed it out with naphtha and alcohol solvents to remove any residual water. Then I coated the inside of the tank with ATF (a nice, thick oil that really coats and stays there and is easily washed out with gas) and some Marvel oil to prevent rust until I use it next.I'm sure "The Works" works but I liked the zinc phosphate coating that the Prep & Etch left behind, as you will find the freshly cleaned bare metal with flash rust SO fast without it.
It seems to stay, from what I have seen, as the inside of my tank is still this perfect dull grey color that it gained from the phosphoric acid treatment. Prior to cleaning, it was more of a shiny steel color (with some rust) but is now universally dull grey. It has lasted now for almost 3 years of near daily driving. I also have used SeaFoam and Marvel Mystery Oil in the gas so it does seem to stand up well even with additives usage, though I'm sure some may have washed off it was probably minimal?Even with our very rainy Oregon winters, there is just not as much as even a speck of rust in the tank. Even up by the neck area, where I see a lot of small spots of rust on otherwise clean fuel tanks on bikes (probably because it doesn't get submerged in gas much) there isn't any rust forming on my tank. I do use the aforementioned Marvel oil which really helps on keeping rust from happening in the tank, but still there isn't a speck anywhere so I think the zinc phosphate coating seems to really help in corrosion prevention.Also of note is that I clean and check the gap on the plugs with every tune and noticed no coloring changes on the electrode compared to before the tank treatment, so it doesn't appear to be contaminating the combustion process, either.Zinc in oil is good, of course (ZDDP - at least if you are running an older engine without catalytic converter) but I'm not sure about doing anything in gasoline - especially in the trace amounts were talking here. Googling around seems to bring up a few mentions of it, including people wanting to actually zinc plate the inside of a motorcycle fuel tank, but nothing that I could find negative. Could it be just be a potential complication if emissions systems are involved? (why ZDDP is no longer in common regular motor oils, as it can contaminate the catalytic converter and O2 sensors, etc.)
Banging stuff around in your tank to attack the stuff inside does work and is easy if not invigorating. I use roofing nails cause they are 'pointy' and some tanks have 'pointy' areas, like my 71 BMW R75/5.
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