Let me offer my opinion on sharpening serrated knives.
First, there are two different types of serrated knives - cheap ones and good ones. The cheap ones have stamped blades, molded handles, a hollow ground back, and usually a mixed pattern of large and small serrations. Think Ginzu. The good ones may have forged or stamped blades, but have a good handle, a flat ground back and only 4 or 5 regularly spaced serrations per inch. Henckels and Wusthof make good examples, although they also make some of the cheap ones.
Second, you will see two different types of wear on the knives - a) the points between the serrations get dull and b) the edge of the serrations themselves get bent or form burrs. This happens because the serrations are sharpened at a very low angle with a single bevel or chisel edge. The result is a very sharp but fragile edge that, although it is protected by the projecting points, does get bent by hitting bones or frozen food.
Cheap serrated knives are not worth more than a couple of minutes of your time. If they have type a) damage, the best approach is to make a single pass over the Tormek wheel on the beveled (front) side, then use the leather wheel to hone the resultant burr off the back side. One more pass of the leather on the front will finish the job. The result is sharp, single bevel tips that will pierce a tomato skin. (I think this is one job where a paper wheel system is better suited than the Tormek.) (Do not do this on steak knives. See the reason below.) If cheap serrated knives have type b) damage, throw them out. This type of serrated knife was originally designed to be disposable, and now is the time to do it.
Good serrated knives are worth doing it right, and that means sharpening each serration as well as the tips. Since we do not want to round off the corners of our Tormek wheel, it is better to do this by hand. (Again, paper wheels would be good here because it is no big deal to sand down a paper wheel to re-square the corners and apply new abrasive. There is also a 1/4 wide paper wheel available that can be used for serrated knives.) Small round hones can be used, but one really good tool for this is the SharpMaker by Spyderco. The SharpMaker is an advanced version of crock sticks which uses triangular sticks. I set mine up with the point of the stick up for the serrated side of the blade and the flat side up for the back side. As you run the knife down along the stick the point it goes into each serration. (You will have to adjust the angle with a twist of your wrist to match the bevel angle because it is different than a standard knife.) Repeat until you have raised a burr, then move over to the flat stick for the back side, holding the knife nearly parallel to the stick to duplicate the flat grind. This will take 5 to 10 minutes per knife, depending on their condition.
One serrated knife that I will not sharpen is Cutco. They have a unique grind that I cannot duplicate, and the manufacturer offers free sharpening. I tell customers to send them back to the factory. I will check them for burrs, and if that is their only problem, I will buff off the burrs, but do not grind the serrations.
A word about steak knives. Many types of knives are sold as steak knives, but a true steak knife has blunt tips. This is to keep the knife from scratching your plate. A serrated knife should be used with a cutting board, not a plate.
Sharpening Made Easy: A Primer on Sharpening Knives and Other Edged Tools by Steve Bottorff
Copyright January 2002 Knife World Publications
Here is a note from Carrol Smith, retired owner of Razor Sharp Edgemaking Systems, the premium maker of paper wheels:
Serrated knives: Some knives have such small serrations, that I do not know how to sharpen, except to treat them like a plain straight blade!...Normal serrated blades, such as Spyderco, I sharpen as follows. I run the flat side of the blade, usually one pass is enough, across the flat of the Gritted wheel, hardly any pressure at all, and almost flat against the wheel. This gives me "burr" on the serrated side, both on the points and in the valleys. I then draw the serrated side, (with the burr), across the corner of the slotted wheel, very gently, then I gently draw the flat side across the face of the slotted wheel. I now have the serrated portion of the blade quite sharp, with no damage to either wheel! Right or wrong this method has been working for me for about 12 years!
Updated November 29, 2006