Draw the cutting edge of the knife against a front corner on a carbide tooth. You do not need to use a lot of pressure. A few repeated passes will do wonders to a blade. The green lines represent the angle of the tooth face and the angle at which the blade is held. The brown lines represent a vertical line and the angle from the vertical at which the knife is drawn across the corner of the carbide tooth. Draw the knife slowly. Hold the blade with two hands to keep the edge on the relatively small corner of the tooth.