And here’s part 2…
#3) Apex
If you felt damage or saw reflections during your examination, portions of the edge are not “apexed”, and you should use a coarse stone to grind away material along the entire edge, (I suggest around 16-20 degrees per side for most knives), until the reflections/dull areas disappear and/or you form a “burr” - see below.
I tend to use a back-and-forth motion while sharpening because it’s faster and I’ve developed good muscle memory, but many people prefer to use only “edge-leading” strokes.
Depending on blade shape, it may be necessary to adjust your hand position while sharpening . On a standard blade with a curved “belly” section, you will need to increase the angle between the stone and the end of the handle as you sharpen the belly and tip areas while maintaining the same angle between the stone and the spine of the knife. (This should be seen to be fully understood).
As you grind away material from one side of the edge, a “burr” or “wire edge” will form on the opposite side. You can detect a burr with magnification or by dragging the burr-side of the edge in a stropping/edge-trailing motion across your finger, fingernail or some paper to feel for roughness. A prominent burr will negatively affect edge sharpness and longevity and should be reduced as much as possible.
If I’m starting with a dull or damaged edge, I’ll do a “full sharpening” which is accomplished by sharpening on one side of the edge until a burr can be felt along the entire opposite edge, and then sharpening the other side until the burr can be felt along the entire OTHER side of the edge.
Once a burr has been formed on both sides of the blade I know an edge apex has been created and I only need to remove/reduce the burr to achieve a sharp edge.
To extend the lives of my knives, I only intentionally form a full burr when re-profiling or sharpening a dull or damaged edge. If the knife has simply lost some of its sharpness, light strokes on a fine stone or strop will often bring the edge back to almost as sharp as a full sharpening will.
Once you have an apex, continue to sharpen with your coarse stone briefly, alternating between sides, and reducing pressure as you do so. Alternating sides and lowering pressure will help minimize the burr and should be done with each stone you use before moving to the next higher grit.
An edge that doesn’t have an apex CANNOT be sharpened!
#4) Reduce the burr/refine the apex:
If you create a substantial burr and fail to remove it, it will tend to fold over when you cut any tough material, resulting in a dull knife.
It’s impossible to completely remove the burr at a microscopic level, however reducing it will significantly prolong edge retention.
Using the same angle as above, move on to your next finer grit abrasive and grind away again, but using less pressure. You can use the method from #3 again, creating a new, finer burr along the edge and then switching to the other side to do the same, or you can ignore the burr at this point and just spend a little time on each side of the edge with each finer stone.
In theory, you should sharpen at each grit until, with a microscope, you could no longer see the larger scratches made by the previous grit. In practice, you should just spend a minute or two with each new stone, ending with very light, alternating strokes.
Repeat this process using stones of increasingly fine grits as desired for a more refined, sharper and longer-lasting edge.
#5) Strop:
Stropping will further reduce the burr and refine/polish the edge.
A strop is a flat piece of leather or other similar material that can be used to remove the burr and achieve a higher level of sharpness in blades – particularly razor blades.
The strop should be used with an “edge-trailing” stroke, so you won’t cut into the leather.
Because leather is flexible, it’s easy to unintentionally round-off and dull your edge with your strop. To avoid this, either use very light pressure at the same sharpening angle OR use moderate pressure at a slightly REDUCED angle (for instance, holding ~18 degrees for sharpening, and ~16 degrees for stropping).
I highly recommend using an abrasive compound (diamond suspension is my preference) on strops. Although a plain leather strop can smooth out an edge slightly and remove the burr, it won’t remove much material if at all.
A strop with compound will remove steel, although at a very slow rate.
I use strops both to refresh slightly dull knives and as the final step in sharpening.
Notes
Most variables in knife design are a balance between desirable qualities. A knife with a more acute edge angle, a thinner blade stock, a harder heat treatment, and sharpened to a finer grit should have a sharper edge that will last longer and cut more efficiently. Conversely, a knife with a more obtuse edge angle and thicker, softer steel edges will generally respond to abuse by deforming with less breaking or chipping.
The steel just behind/underneath a very dull or damaged edge is likely to have been weakened through work hardening. These edges may benefit from a fresh start, whereby the edge apex is ground away by gently running it perpendicular (or close to perpendicular) to a stone a few times (and then checked for a solid line of reflection down the edge). This process removes the stressed/weakened steel at the edge and reveals tougher steel underneath.
Many factory knives come with edge angles of significantly greater than 20 degrees per side, which is less than ideal as it makes them more difficult to resharpen and reduces edge retention.
A 20 degree/side edge is generally considered to be tough and still easy to sharpen. High toughness steel knives can chop dry hardwoods without damage at even more acute edge angles.
High precision sharpening is generally slower and requires more mental effort but removes less material and can usually produce a sharper edge at a given angle. High precision sharpening will also generally lead to better looking results. Precision can be enhanced by using “fixed angle” devices or other sharpening systems (although these also require some practice). I sharpen most of my knives with comparatively LOW precision to save time.
The tips of knives are typically a little tricky to sharpen. To avoid rounding/damaging the tip, try to stop the sharpening motion while the tip is still in contact with the stone (rather than allowing the tip to slide OFF the stone).
Because blade geometry (the 3D shape of the blade) often results in more material at the tip for strength, it may be necessary to spend more time grinding at the tip than along the rest of the edge.
Unless a blade has been ground unevenly, the edge bevel on each side of the blade should be the same width if sharpened to the same angle. For instance, if you have a thinner edge bevel on the left side of a knife blade, that generally means that the left side edge should be ground down more (at the same angle as the right side) until it matches the right side.
Occasionally factory edge grinds (or even primary/main grinds) are not properly aligned, and that’s something to look for when purchasing a new knife. Symmetrical grinds from the factory/bladesmith will save you time and effort in the long run.
While you should concentrate on trying to maintain a consistent angle while grinding, the slight human error inherent to freehand sharpening creates a convex edge and automagically thins out the “shoulders” of the edge with each sharpening – both of which are generally considered to be good things for edge stability and cutting performance.
Embrace your wobble!
