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You can get a good shaving edge off the 8k, though the 15k is awfully nice. But I agree that it's complicating things for you - you need to get a handle on the 4k and 8k before you worry about the...

Sounds like you overhoned it, then broke the wire edge off on the 15k. Go back and do a minor pyramid on the norton, if it's sharp then do 10 laps on the 15k and test it again.