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I believe you can buy Rottenstone wherever woodworking supplies are sold. (I usually buy from Lee Valley Tools and Woodcraft.com...I'm pretty sure both of those places sell it)
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Oh, ok. Rottenstone is sorta like a Nagura, used for making slurry. I don't think it actually cuts, itself. Just makes the stone cut faster by making a slurry. Bill may be doing something...

I haven't seen the video, but might it not be lapping compound, to get the stone flat? If so, it doesn't stay on the norton, but rather, you wash it off.