Phosphoric Acid Rust Removal

Phosphoric Acid is used on ferrous metal (steel, iron) to remove rust and coat the metal to protect it from further oxidation. It also has the ability to lightly 'etch' the surface for better paint adhesion. But mostly for the rust stuff.


There is a huge explanation on how/why it works... but for the most part, it simply works. Most shade tree mechanics will equate Phosphoric Acid with molasses, vinegar, coke etc... and they are right.. those products contain phosphoric acid in some diluted quantity. The most readily available forms of phos. acid are in cleaning products. CLR, tile/grout cleaners, or even simply marked 'Phosphoric Acid Cleaner' will contain more phos. acid than we need for our purposes. The phos. acid put out by Aquamix (available at Lowes) is 23% phosphoric acid by volume... pretty good stuff. You can even pay for expensive 'metal prep' products from auto body stores (made by 3M), but there are nothing more than fancy bottles of phos. acid.
Here is a picture of a piece of metal with the some light/moderate rust. It's a steel M-122 tripod leg. The parkerization on a weld has been removed and the steel patch has rusted. This is a typical rust spot that you would see on a jeep panel, or any metal object. The rust is not pitted, but more prevalent that flash rust from sitting water.


This picture is after less than 3 minutes of the phosphoric acid being applied with a paintbrush. The shiny steel is now a dull gray (that's the protective coating left by by the phos. acid) and the rust is clearly gone. The rust from around the tripod's foot as well as the leg is completely gone and protected from further rust. The part was rinsed under tap water to neutralize the phos. acid and even after air drying no flash rust developed. Add a coat of primer and you're done.

Here's a more jeep specific part... the rear hat channel.. This part runs under the rear floor and acts as a body stiffener. I don't have a 'before' pic.. but this shows all the phos. acid goodness.... no rust. Even in the drain dole in the middle of the picture. The previous owners weld 'globs' where rusted and now are completely free and a nice dull gray, protected from further rust. Grind off the weld globs, a coat of primer, then Red Oxide, then O.D. and you have a clean, almost new channel to weld back onto the jeep.

Total time coated with phos. acid was less than 5 minutes.


If your rusted part has advanced to the pitting of metal stage, you've already lost as that metal can not be replaced. You can still use phos. acid to remove the rust.. it will do a very good job of getting into the pits and cleaning out all the nooks and crannies... but the metal is gone. Media blasting will clean the part and also remove the pits, as will any mechanical rust removal. But on jeep panels that are thin to start with this further removal of material may cause the panel to completely fail (ie you now have a hole in the panel instead of a rusty panel).