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“The more you sharpen the blade the more it deforms, and instead of a straight edge line, you end up with a concave curvature that develops towards the bolster.”
In my classes, we use marking/carving knives quite a lot. We score lines in preparation for making a “knife-wall”, we use them to whittle animals, to cut paper veneer and more.
Our knives are shaped in the traditional European pattern with a wide bolster at the meeting point of the blade and the handle. To sharpen a knife I use DMT diamond stones. Typically, I begin with a fine stone and move on to extra fine and finish with a strop.
Over time, I noticed that the rims of the rectangular stones lose many of their micronic diamonds and cannot erode the blade efficiently. That caused a problem, which eventually alters the geometry of the blade. The more you sharpen the blade, the more it deforms and instead of a straight edge line, you end up with a concave curvature that develops towards the bolster. Curvature in a blade edge may not be that big of a deal if you are willing to spend the extra time and care to sharpen and hone it, including using round sharpening medium on the concave area. But attempting to achieve this solely on a flat diamond stone is problematic, and will eventually reduce the effective length of your cutting blade to only the straight portion of the edge.
A typical European style marking or carving knife has a wide bolster at the base of the blade.
But over time I noticed that the rims of the rectangular stones lose many of their micronic diamonds and cannot erode the blade efficiently in the area near the bolster.
My solution to this problem was to create a gullet at the meeting point of the blade and the bolster. This is not my innovation of course, it’s a geometry that I’ve seen in other knives – though much bigger. To create the gullet, one can offer the blade to a grinder and use the corner of the grinding wheel in order to cut into the steel.
Another option is to use high-speed rotary tool equipped with a narrow cylindrical or conical grinding stone. And of course, there is the option of using a diamond file or sandpaper wrapped around a nail or a dowel. 
Once the gullet is created, sharpening becomes fast, easy and repeatable. It will ensure that every time you sharpen, you’ll get a long and uniform straight edge. Over time, you will need to revisit the gullet and deepen it, but that effort is well worth the time spend.
– Yoav Liberman
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