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Before going through a car wash, I took my mobile antenna off its NMO mount and it looks like water seeped in through the threads. It's not corroded (i.e., there's no metal flaking off) but it's not clean (a layer of oxide.) I was thinking of using some Barkeeper's Friend to get the oxide off, but before I do, how do I clean the oxide off? Dirty NMO mount

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    $\begingroup$ This isn't the question you asked, but it might be useful to point out that a NMO mount is essentially a large coaxial connector, and you shouldn't get it wet uncovered because water might seep into/around the dielectric and stay there, and even get into the attached cable, depending on how the mount is constructed; this one might be able to drain out the bottom, but a magnet-mount or through-the-roof mount would be at great risk of internal corrosion and moisture. I'd suggest getting a cap that you can install on the mount whenever you remove the antenna. $\endgroup$
    – Kevin Reid AG6YO
    Apr 7, 2022 at 4:07

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That's called "flash rust", and it was caused by both the water and the strong alkali detergents used in car washes. Similar to how an aluminum pot will flash rust when run through a dishwasher.

It can be removed with products you can buy at the hardware store (e.g., "Evapo-Rust", "Iron Out") but even hydrogen peroxide or vinegar (and a softer scrubbing pad or old toothbrush) will work for light flash rust. Be gentle, as you don't want to remove actual metal, just the surface oxide.

Once you have removed the oxide, clean the mating surfaces well with a solvent suitable for electronics (again, check your hardware store for "contact cleaner"). The idea is to displace the water and dirt so you have a clean surface.

There are greases suitable for electronics that you can then apply immediately to prevent flash rust. There is an electronics spray used to clean potentiometers that leaves behind a light coating of lubricant that would probably work, and can be used on your stereo and Amateur equipment as well. In a pinch you can probably just use WD-40. I'd spray it onto a cloth and swipe the metal surfaces to give them a light coating.

Though a proper electronics grease will really protect the electrical mating surfaces from any road grunge over time.

You don't have to use an electronics grease. But a swipe on the threads would not go amiss. Don't slather the stuff on, of course. It conducts electricity. So treat it like you would solder.

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