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Knowing when to stop is generally important, even with other stones. With the pros, pressure is the nemesis. They literally are a case of "let the stone do the work".

Nahh they're both yellow. ;)
Seriously tho I have only used an SS12k a couple of times so can't really say but the Shap is certainly one of the softer stones in that set. I used to jump 5k -12 k...

Maybe the op can weigh in on this but all I can think is maybe the 12k is leaving fine scratches that become visible on the polished edge off the Norton. The Norton having a more friable abrasive...

The higher you polish without a good foundation the smoother the edge gets but perceived sharpness drops if the 2 sides of the bevel don't meet optimally. The problem is earlier than 8k or 12k. ...