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The coticule had little to nothing to do with it....after 600 passes you should have achieved a convex microbevel...the angle you hold your blade should have increased for optimum cutting...the edge...

Oil and finishing under water help indeed but not so if the stone is inconsistent.

Page 3...pictire of the stone wet evidences a transverse line light redish-brown in colour.

That could be a line of iron that is/has been transforming solwly into red iron oxide-aka rust.

Rust...

Lapping wont do it any good.

If you feel the drag is changing when the blade hits the line...in the way thet the drag increases and you get a harder audio feedback....you may run into a patch of...

Oh...your post made me smile...
I have a hunch I know what is happening.

You have bothe the 12k stone and the coticule near...please check to see if your hones are flat.

Sometimes it happens...

Oh...beautifull coti.
That's a La Grise form the Ol'Preu quary.

Should be
Medium hard or a bit on the soft side....
Moderatly fast on slurry.
Slow on water only.
Provide a realy nice and...