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Good point, that black shmootz on Hirlau's Norton 4k is fairly reminiscent of the crud on mine but the only action my hone has seen is clean razors and water, so in my case it's metal particulate....

That is some incredibly determined oil residue. I figured a simple green soak like I gave my Arkies woulda fixed the issue. Tossed 'em in a bucket mixed heavy on the SG and let 'em soak a few days...