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Lap it flat on the 220 then 10-20 laps on each of the finer grits.
The 400 grit finish should be just fine. Be sure to use the sandpaper wet and do not use much pressure. To much pressure and some...

Be sure to check the flatness of the DMT. I have seen one that was not flat.

To keep the metal filings from building up you should be sure to use water on the hone while honing. Not a lot, just a spritz and whenever you see that metal residue, called "swarf", starting to...