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If you read the two threads I linked it will give you a good base understanding, after that, there are several theories that come into play...

Which of them are really true I am not sure of.. I...

I use a well worn DMT 325 on most, however for a true cutting slurry on the Norton 4k I use the Norton Prepstone... There are vids of all this, and threads about much of it too..
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Use both now, and have used both for years, in practical application I find both to work excellently

If I were to only own one of them I find the Norton to be more versatile in my hands..
I...