Fishing is a very popular sport here in New Zealand. A filleting knife is an essential item if you are wanting to harvest some fish for the table. How to sharpen a filleting knife is a skill that is lacking with most fishermen.
A filleting knife works best with a very sharp blade. This requires a shallow bevel angle of approximately 15 degrees. This angle can be measured with most smart phones. We have some more detailed information on measuring knife bevel angles in our knife sharpening blog. For the sharpest edge on you knife you need to use a very fine grade stone. The Belgian Coticule Whetstone is the sharpener of choice for filleting knives due to its very fast and fine sharpening properties.
A filleting knife needs to have a flexible blade. This allows the edge to move over the bones of the fish rather than trying to cut through them. A good quality knife will flex evenly across its length and slide over bones without damaging the sharp edge. A cheap knife will flex from one spot just in front of the handle, this type of blade will try and cut through bone and becomes dull quickly. The flexibility of a filleting knife can make it difficult to sharpen the last half of the edge. Here is a technique that works to keep such a flexible blade in control when using a Whetstone.
It is easier to hold a stable angle by sharpening the knife in four sections. Start with the heel area, then the middle, belly, then tip. The key difference between sharpening a filleting knife compared to a kitchen knife is that the last half of the blade is supported with the fingers or by using two hands. Finger pressure is applied to the blade to keep it straight and in full contact with the Whetstone. Hold the knife off the stone at your predetermined angle. Feel for the position and lock your wrist.
Start at one end of the stone and move the blade in a sawing motion, working towards the other end of the stone. Light to moderate pressure is all that is required as the abrasive surface of the stone will do the work for you. Do 2-3 passes (lengths of the stone) on each side of the knife then move to the next section of the blade. Test the edge with your thumb regularly. Get used to how the edge feels as it becomes sharper. Concentrate on the angle. The belly section and curved tip will require you to rock the knife slightly due to its curvature.
We have another technique for sharpening filleting knives using a circular method which is also in our blog section.
A properly sharpened filleting knife will shave arm hair with out much effort. A shallow bevel angle and fine sharpening stone are required to achieve this level of sharpness. Contact us if you would like to visit for a knife sharpening demonstration in person or via Skype.