You can sharpen a serrated bread knife, but not with the whetstone used for a straight-edge knife: the cutting edge is the concave inner face of the gullets between the teeth, and only a tapered sharpening rod that fits into a gullet can reach that surface. If you sharpen a serrated knife on a flat stone, the teeth are filed down from the top, the gullets become shallower, and the knife can no longer do its job as before.
The cutting surface on a serrated knife differs from that of a straight-edge knife. The pointed tips of the teeth only pierce and grip the crust; the actual cutting happens at the edge of the concave gullet between two teeth. These gullets are cut into only one side of the blade; the other side is flat. That makes the serrated knife asymmetrical, so sharpening must also be done asymmetrically: work only on the side with the gullets and barely touch the flat side. This asymmetry also depends on which hand you use; a knife with the gullets facing right is harder to use left-handed without leaving marks.
A flat sharpening stone does not fit this geometry. Because the stone contacts the edge all at once, it wears down only the tops of the teeth and cannot enter the gullets. After a few passes, the teeth are shorter and the gullets are shallower, turning the knife from a saw into a poor straight-edge knife. It then starts crushing and tearing bread crusts and slipping off tomato skins. This damage cannot be undone; the teeth cannot be made deep again. Electric sharpeners cause the same damage faster because they treat the edge as having a single geometry and grind down the teeth from above.
The right tool is a tapered sharpening rod that gradually narrows toward its tip. Find the section of the rod that fits snugly in the gullet, keep the gullet’s own angle, and make a few strokes outward only, not back and forth. Work on each gullet individually, using the same number of strokes; pressing too hard on one gullet distorts the edge. Finish on the side with the gullets, then remove any burr from the flat side with just one light pass.
The surprising thing is that a serrated knife rarely needs sharpening. A straight-edge knife touches the cutting board with every cut, dulling its edge there; on a serrated knife, only the tooth tips touch the board, while the cutting gullets never contact the surface. That is why in the same home kitchen, a chef’s knife may be sharpened several times a year while a bread knife lasts for years. If it is not cutting, first question how it is being used, not the edge. Most bread knives need cleaning and proper storage more than sharpening; a knife rubbing against other tools in a drawer loses its tooth tips.
The most common culprit when a serrated knife does not cut is dried bread and sugar residue lodged between the teeth; when a gullet is clogged, the knife behaves as if it were dull. Soaking it in warm water and cleaning between the teeth with a soft brush is often enough. The second culprit is denting at the tooth tips: when the knife is forced through something frozen or very hard, the tips bend sideways. A truly worn serrated knife should be replaced rather than sharpened. A serrated knife with bent or dull teeth is an inexpensive tool, and replacing it is often cheaper than having a professional sharpen it.