I’ve written a few words before about removing rust with acid, and the last week I made a simple experiment with citric acid.
The target are the mounts for the fog lights that come with the special “GT”-grille. They’re made of steel and were painted flat black when new. As expected, not much of this paint was left and the mounts had sections with really deep rust.
First thing to do was to remove all kind of dirt and grease. I also use a rotating brush and an old knife to remove most of the paint. The paint removing step would later show being totally unnecessary.
I took a plastic bucket filled it halfway with water, I guess around 20 liters (5 gallons). It’s important to use a plastic and not a metal bucket, watch the first episodes of “Breaking Bad” if you don’t believe me. I then poured a little more than half a kilo (one pound) of citric acid that I bought from a grocery store in the bucket. As they taught you in school, always pour the acid in the water and not the other way around. My estimate is that the acid concentration is around 5-10 %, and I’ve heard that stronger concentration doesn’t do any difference. Finally, I put the mounts in the bucket and also added some frames that came with the fog lights.
Since the acid bath only covers half of the mounts, I had to do it in two rounds. Unfortunately I had to go away, so they were left in the bath for a total of 100 hours. They came out completely rust free, and I honestly believe 48 hours would be enough. The temperature in the garage is around 23 degrees celsius. I’d also like to add, that when you take them out the bath, they’ll probably look a bit rusty. However, this is just a coat of “dead rust gunk” that can be wiped off! Another interesting thing is the low quality factory paint that was still partially there, also could be wiped off in most cases, or at least by scratching with a finger nail. You could by the way use vinyl gloves, however, the acid isn’t that strong so it’s harms your skin in shorter time periods. Just wash your hands or whatever body part you’ve used when you’re done.
It’s important to thoroughly rinse off all citric acid after the bath is complete. However, since the metal is so clean, it will start to rust within minutes. Some kind of surface protection is required. Don’t touch the bare metal with your skin, even though it’s tempting.
My main method is to wipe it clean with a paper towel and apply a layer of phosphoric acid (or any of the popular “rust converters” that contains phosphoric acid). This will create a phosphate layer to protect the metal from moist, but it will also go deep into the pores and take care of the moist from the rinsing. The parts can now be safely stored for a while in a dry environment until I paint them with a 2k epoxy primer.
Another method is to quickly dry the parts with a hair dryer or torch and then immediately spray them with a primer. I personally don’t like this as it’s not sure that all moist in the pores will be deleted. One alternative is to wipe them with alcohol, since alcohol takes care of moist.