How to sharpen a wood carving knife
This post will answer how to sharpen a knife. In particular I will detail sharpening a curved wood carving knife, such as the single bevel double edged bent knives that northwest native American carvers use as these are most difficult to sharpen and require special curved sharpening tools, but the techniques described here will work equally well on all knives, such as double bevel straight knives.
Sharpening a knife is something that must be done carefully. Not only is there the risk of accidentally cutting yourself, if not done correctly you can do damage to the knife and sharpening tool. Wood carving knives need to be extremely sharp to cut properly and not cause wood fibers to tear out of the wood.
I recommend for that reason that you first practice on knives you don't mind damaging. If you don't want to take the risk of damaging your best knives you can always send your knives off for professional sharpening. Even if you do sharpen your own knives I recommend sending them off for professional sharpening occasionally to get the bevels ground flat again
Tools to use
First let's go over what tools to use. I prefer sandpaper, diamond laps and polishing compounds.
Sandpaper works well for doing initial flattening of rounded bevels and removing metal to fix nicks and broken tips. Sandpaper can be taped or glued to dowels or boards to sharpen both curved and straight carving knives. When using sandpaper only sharpen away from the edge or the blade will cut into the sandpaper.
Diamond laps are hard and come on a wide range of grits. They cost more but essentially last forever and never get gouges worn in them. You can sharpen the knife in either direction on a diamond lap.
Polishing compounds, such as green aka chrome oxide (fine, about equivalent to 2000 grit) and black aka emery (Course, about equivalent to 800 grit) are wax based and are very good for final sharpening. Diamond based polishing compounds can be had cheaply in assortments of 0.5 to 40 micron. Note that microns work in the opposite direction of grit, and 0.5 micron is equivalent to 60,000 grit while 40 micron is about 400 grit.
Polishing compounds can be applied to a number of materials. Some of my favorites are wooden dowels and poster board. Care must be used to only sharpen away from the edge or the blade will cut into the base material.
What not to use and why
Leather strops: Leather is too soft for carving knives IMO. it will slowly round the edge of your knife over with every stroke. This does make it very easy to use as holding the knife at too shallow or steep of an angle it will still sharpen the tip of the bevel. Leather strops with no compound are however very useful for removing a wire edge after sharpening by working the wire edge off.
Oil and water stones: Some claim stones are great, but they all require care to remain flat and generally need water or oil added to prevent clogging up, increasing mess and deceasing how often you sharpen your tools. You need a number of them to cover your range of grits needed and the price adds up fast! its hard to find the same style of stone in a wide range of grits and the cutting speed is often not that great.
Diamond laps are generally cheaper, cut faster and are way more durable then sharpening stones. Making your own lap out of wood and lapping compound or using sandpaper is way cheaper still but not as durable and can only be used in one direction, but cheap enough to throw out if it gets damaged/worn out and can cheaply have a wide assortment of grits and shapes required to sharpen carving knives and gouges.
Techniques, tips and tricks
Now for the sharpening, I recommend using a Sharpie brand marker and painting the bevel. Many other brands don't work on well metal, Sharpie is the best brand of marker I have found for this task.
A painted bevel let's you monitor your progress, bevel flatness and knife angle by looking at where the ink is worn off. Repaint the blade frequently to see if you are holding it at the right angle. When you can take off all the sharpie marker from the top to the bottom of the bevel without changing the angle of the knife, the bevel is flat.
You do not need a perfectly flat bevel, but trying your best to keep it flat will keep you from increasing the angle of the cutting edge by rolling over the end. a knife is not sharp till you have removed the sharpie marks from the cutting edge of the blade with each grit you use.
One should resist the temptation to simply angle the blade and sharpen just the very edge of the blade as every time you do this you will round over the cutting edge making the blade effectively duller due to the wider bevel angle.
For northwest coast style double bevel curved blades, use a dowel with sandpaper taped/glued on or with lapping compound applied to sharpen the inside curve of the blade. The dowel must be smaller then the smallest radius on the knife so I recommend having a selection of different sized dowels as you should also use the largest dowel you can on a knife. Use sandpaper, diamond plates or lapping compound on a flat surface to sharpen the outside of the curve.
I recommend around 5 strokes between checking the blade and repainting it with sharpie while you are learning to sharpen. Eventually you will only need to repaint it when you change grits.
With sandpapers and soft lapping materials you will needs to stroke away from the cutting edge while diamond sharpeners and stones can be used in either direction.
The most important part of sharpening is to maintain the same angle consistently. Consider placing your finger behind the blade to press the bevel into the sharpener but be careful not to cut yourself if you do.
Grit selection and progression
For keeping a blade sharp, generally something like a 1000+ grit diamond plate or sandpaper, green polishing compound or 10 micron diamond paste works very well while leaving a good finish and may only need a few strokes to resharpen the knife.
If a blade has been dulled or was not very sharp initially, you may want to start off with a 600 to 800grit diamond plate or sand paper, black polishing compound or 25 micron diamond paste.
If you have to reshape the blade and remove significant amounts of metal, it is recommended you start with something like a 200 to 400 grit diamond plate or sandpaper or 40 micron diamond paste.
You should also not skip grits or it will take longer to remove all the scratches. that it not to say you should buy every grit available but decide upon a decent progression.
For example I recommend:
Grit: 200, 400, 600, 800, 1000, 2000
Micron: 40, 28, 20, 14, 10
Note that with micron rated lapping pastes, a lower number is finer, the opposite of grit rated abrasives. Going above 2000 grit or below 10 micron won't arguably make a knife any sharper but it does give it a much nicer polish. Diamond pastes are your best bet if you wish get a mirror finish, with pastes commonly available to 0.5 microns leaving an absolute mirror finish, equivalent to 60,000 grit.