No, it is silver paint using the 'mustard mask method'.
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I didn't bother showing the process since it has been around for years now and
@OddJob recently did a good job of explaining it again.
However, I did discover something unexpected this time.
When I first started using this technique I had seen it used by model car builders but with Vasoline. I wanted to use something that was water soluble and after searching around the house I landed on mustard. It checked most of the boxes that I had in mind; cheap, easily accessible, and high viscosity that would stay put. The yellow color was a bonus as it made it fairly easy to keep track of the application.
I said all of that to say that it could have just as easily been anything else but it just happened be what I found first. Fast forward to now and it had consequential effects.
Up until now I have applied the mustard over a painted base coat because I'm usually making fiberglass look like vintage metal. This time I started with real metal that had been chemically 'browned' which is basically controlled rust. As soon as I started removing the mustard mask I realized what I had done. The first ingredient listed in mustard is vinegar. Vinegar is acidic which removes rust!
The result was that some of the brown metal had been cleaned by the mustard back in the direction of silver/grey metal.
Luckily the silver paint was fast drying so the acidic effects of the mustard didn't have enough time to completely wreck the base color. In some ways it was a happy accident in that it created a variegated color that has a natural look. The only downside is that it is not as dark brown as I had envisioned but not enough to make me refinish it.
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Onward!