David Weaver
At one point, Spyderco mentioned that the particles in the stones was 3-5 microns or something of the sort. I don't remember if that was the UF and F and not the medium, but it really doesn't matter. I think they had said that the UF and F were the same material, but the UF had a finer surface finish.

The magic is that they hold the particles in place and allow them to dull, and there's little or no air in them, so the particle size isn't critical.

I had used barkeeper's friend for a little bit and done some other things to keep the white stones clean but never had an issue with those, just the brown medium, which is more of a medium/fine stone.