For edge repairing
For reparing an edge that has larger chips in it, a lot of material needs to be removed. The easiest way is to give the knife to a professional sharpener, who will use machines (rotating whetstones or belts) to do this.
For home sharpeners, it requires very coarse grinding mediums, like a coarse whetstone with a grit of at least 400 or lower, or (if available) a belt sander, belt grinder or Tormek.
Here the objective is to obtain a straight edge again without chips. Once this is achieved, the blade can be sharpened normally again.
For edge sharpening
The easiest way is to give the knife to a professional sharpener. However, a rounded edge can also easily be sharpened with different home sharpening solutions. All of them have in common that you move the blade against a harder medium at a defined angle, thereby removing material from the blade on the cutting edge.
Pull through sharpeners
Those are the easiest sharpening solutions available, divided into two types:
Roll-sharpeners, where the edge is pulled over vertically arranged round rollers or discs in a 90 degree angle. There are roll sharpeners with metal or ceramic discs / rollers, and with 1 or more rollers for rough and fine sharpening. Neither do roll-sharpeners produce beautiful edges on a microscopic level, nor can they restore an edge to a top level sharpness. Still, for casual users who want a cheap and easy sharpener that just does the job of making the knife cut again, they are a good solution, definitely better than keeping the knife dull.
V-sharpeners, where sharpening elements are arranged in a V-shape, and the edge of the knife is pulled at a 90 degree angle through the V. There are different sharpeners with varying sharpening results, from easy and inexpensive sharpeners that do an ok job, to semi-professional solutions like the ZWILLING V-Edge that can produce a good sharpness and with a clean, nice edge.
It's probably the most commonly used sharpening device and included in many knife block sets. Often referred to as honing rod, the main purpose is to straighten a bent edge (extensively covered under "Edge Maintenance"). But depending on the coarseness and hardness of the rod, it does have a sharpening effect.
For the process to work properly, the rod needs to be harder than the knife. It’s important to note that most sharpening steels are in the hardness range of HRC 61-64. Thus they are not recommended for hard knives with high HRC ratings (HRC 61 and above).
Sharpening steels are available in different materials and coarseness:
Sharpening steel with no or smooth ridges: the smoother the ridges are, the more the steel straightens the edge and the less material it takes off. By far the most common type. Usually not suitable for harder knives (HRC >61).
Tungsten carbide rod: made of steel with tungsten-carbide coating. Harder than a honing steel; can be used on high HRC knives. Tends to be a little more abrasive than honing steels, and thus takes a small amount of material off the edge.
Ceramic rod: made of ceramics. Generally more abrasive and taking material off the edge; sharpening effect. Can break in case of inappropriate use.
Diamond rod: usually a steel rod with Diamond coating. Most of the time, the coating is rather abrasive and can remove substantial amounts of metal. More a sharpening solution than for edge maintenance.
The knives are sharpened or honed by holding the rod in one hand, and pushing the blade down the rod at a steady angle (same as the cutting edge) with the other hand.
Whereas professional chefs are often seen sharpening / honing their knives on a rod in quick motions, seemingly hitting the rod hard with the blade, this can only be done with knives that use softer steel at a wider edge angle. It’s not recommended with knives that use hard steel and have thin edges, where hitting the blade too hard on the rod may lead to severe chipping. This is the reason why for Japanese knives, using sharpening rods is discouraged. However, if done carefully, just pushing the blade down the rod with moderate pressure, the risk of doing damage to the edge is very low.
Guided sharpening systems
Since the key to a good sharpening result is holding the knife in a steady angle to the sharpening medium, there is a growing number of guided sharpening systems that allow sharpening the knife with different grits at an adjustable and stable angle. They are very popular among custom knife makers or knife enthusiasts, often for outdoor knives. The recommendable systems (e.g. TSPROF, Edge Pro APEX) are priced in excess of $ 200, quite large, a bit complicated and need a bit of practice, but can produce very precise edges with very high sharpness.
Recently the market has been flooded by clones of those systems (e.g. RUIXIN) at a fraction of their cost. Although they do work in a sense that they can sharpen a knife, they are not as sturdy in built as the more expensive originals, so that it's difficult to keep the angle steady and produce a precise and truly sharp edge.
Electrical knife sharpeners
There is a variety of electrical knife sharpeners in the market in different price ranges. The principle is similar to manual sharpeners, but with rollers/belts electrically driven for quicker removal of material.
They produce acceptable to good sharpening results, but include the risk of knife damage in case of improper use.
Electrical knife sharpeners are an option for home sharpening of more seriously worn edges, but don’t produce better and sharper edges than many manual sharpeners that are easier to use.
Although it needs a bit of practice, sharpening on whetstones by many is considered as the best way to sharpen a knife and that can achieve the best sharpening results. This is why we have dedicated an entire section to whetstone sharpening.