Rust:
http://www.runtimedna.com/forum/showthread.php?t=33166

I wrote about dust years ago but the search engine isn't finding it.

For dust, you mix your shader with a small amount of dry diffuse white, controlled by the Y value of the surface normal, same as we do for procedural snow.

This is very easy in matmatic, not so easy with a metallic shader done by hand.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)