I don't know too much about gun care, but what about doing it in actual gun metal blue? I agree though, it's gotta have some sort of finish or it will rust up on you pretty quick. Or is it aluminum?
Nice work. Just make sure you keep your laces tucked in
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Blued guns/metal will rust faster than most anything else.
If you want to borrow from the gun industry, I'd suggest parkerizing using the cheap-o D-cell battery method- you can get a deep black park using a few simple parts and junk from the hardware store. It does require heating acids in a trough or bucket though- nothing dangerous as long as you don't drink it