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Mate , maybe an angle grinder might be the go. Just kidding but surface grinding something that hard would be pretty frustrating to do by hand. Don't blame you giving it the flick.
Just thinking...

Jimbo, Did you get a layer of pits or sand inclusions ? Obvious question but have you tried lapping another 1mm past the crap layer ?

tbs is spot on. I've listened to So wax lyrical about various stones he showed me & it was like studying the surface of a Nihonto. They do collect for aesthetic reasons but the one Hatanaka stone...