For those who are new to the world of knife sharpening, often the most challenging part is knowing when to stop. How do you know when the knife is sharp enough? How do you get it to that point?
When sharpening a knife, sharpen until you see burrs. Then, turn the knife over, and sharpen the other side. The honing process will remove these burs, but they are important. Burrs are the only way to know that you have removed enough of the metal to sharpen the blade.
Electric Knife
Once the blade has been sharpened using the rougher grit of your sharpener, stone, or file, then you will need to refine the blade. This is usually done with the opposite side of the sharpener, which has a smoother grit. Honing the blade can also be done with a leather strop. Simply repeat the process of sharpening the knife with this finer grit.
Some experienced knife sharpeners test the sharpness of the blade against an object. These may range from cutting across a nylon paint brush, to shaving the hair on your arm, to testing it on your thumb. If testing against your thumb, hold your thumb parallel or perpendicular to the blade and carefully slide the pad of your thumb across - not down - the blade. It can take quite some time to get a feel for this method, however, and you should not try it as a beginning knife sharpener. If you do it incorrectly, it is possible to slice your finger open. Another test is to hold the edge lightly against your fingernail, at about a 30 degree angle. Press down lightly. If it begins to cut into your nail, it is sharp. If it slips, the blade is dull. This can also be done using a pencil, a plastic pen, or another relatively soft solid object.
These tests, however, will not determine whether the edge has a burr or other imperfection. Burrs are naturally caused by the sharpening process. They are thin projections on the very edge of the knife. Usually, they are pushed left or right due to pressure in the sharpening process. Sometimes they point directly off the edge of the knife; this is called a "wire edge." Because the burrs are very thin, this may seem like a very sharp edge. However, it is too thin, and the first use of the blade will break it off, leaving you with a very dull knife. If you seem to be getting very sharp knives after you use your knife sharpener, but they become dull within just one or two uses, this may be the culprit.
The easiest way for beginners to see burrs on the knife edge is to hold the blade up to the light. The light should bounce off the blade uniformly, except for where there is a burr. It can also be felt using your fingernail, although again, you should avoid using your fingers to test the blade of the knife unless you know you will not cut yourself.