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Buildup is a result of metal coming off the razor, it doesn't tell you anything other than that the hone is cutting.

As for your other questions - the best way to find out is to try.

Don't worry about speckles in Naniwas. It is normal. Did you grid the hone and remove the grid? That is what counts. Based on the suction (I believe that is cohesion and adhesion, not capillary...

Honemeister needs a 10 inch XC/C? I don't think so at all, and I'm not sure what would give you that impression. Once you initially lap your hones, all subsequent lappings (if they are done...

Relative size doesn't matter, so long as your lapper isn't tiny. I use a DMT6C to lap my 8 x 3 hones with no problem.

The only way to know if a hone needs to be lapped is to put a pencil grid on...