Since I don't know the math, this is just an ignorant suggestion that ... perhaps could send you in the right direction ... if by accident, could you use a variation of your rust on metal shader?  LIke I said, shot in the dark, probably unrelated.

I am doing that, exactly. The difference is rather than modulating the concentration of corrosion on a UV rectangle (used on a door) or on a hand-drawn UV-based map, I'm trying to discover, entirely within the shader, which are the crevices and which are the protrusions.

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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)