I've done quite a but of hardsurfacing over the years and hardsurfacing rod isn't designed to use as a build up material. That said, depending on the tooth, many teeth aren't designed to be built up either. Basically the alloy they are made of isn't easily weldible, and if you don't use the right rod, right temps, etc through the process the built up area will break off the first time you put stress on it. In other words so your best bet is probably going to be to replace the teeth. Once you have the new teeth in place then they can often be hardfaced the hardfacing only lays on the surface. To be honest though, with the cost of the hardfacing rod/wire/etc, (the last rod I used was around $8-$10 per rod) and the time spend doing it, it's usually cheaper in the long run just to get new teeth unless your running a HUGE machine. That said, most welding mfgs that offer hardfacing rod also have filler rod that they recommend for use under thier hard rod. Personally everything I have worked on thus far that needed buildup, etc was origionally mild steel and I used a regular 7018 rod/wire to do the buildup work. Once I got things like I wanted them I then layed either one or two layers of hardfacing, depending on the rod type and/or wear resistance needed in that particular area, to get the protection wanted.
Hope this helps. Wayne