Well, in talking to Joe at KalGard, he says that you lengthen the time (2 or 3 hours) and the finish is "about" as durable. I can't really testify to the truth of that statement. I figure however, that fibreglass or wood stocks are by nature softer then metal and prone to denting or scratching easier because of this property. GunKote is flexible to a certain degree and "resists" denting/flaking but I don't think it will really hold up on a stock any better then say, your typical Krylon paint.
You can do it in order to better blend your colours with the metal, but I don't think that it will have any more efficacy then just spraypainting with just about anything......