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There are double-sided D6's (the 6" long Diasharps) and all Duosharps are double-sided (like the 8"x2 5/8" W8's) but no double-sided D8's (8"x3" Diasharps). Check the DMT website and see what you like most:
I only use the 600 if there's some kind of damage on the razor, or when I'm working on a very convexed bevel and the particular Coticule I'm using is not fast enough to correct the bevel after 15 minutes of work.
After the 600, I indeed perform a downstroke on glass and start on the Coticule. I know that it take about 2 sets of halfstrokes to undo that downstroke, so I generally don't even bother to check, but dive straight into Dilucot.
i have the dmt 325/600, which bart recomended me. i will second that you can shave arm hair of both grits. i fiond a thew lighter strokes at the end thentest on arm hair and it works. .