The only reason anyone would apply vehicle undercoating today is that they have fallen for the favorite line of this industry, which is that all vehicles come out of the factory unready to face the rigors of ice and snow. Today, nothing could be further from the truth. Not only is automotive metal specially treated before it is painted, it is dipped in an electrophoretic bath about half-a-dozen or more times before it is put into the sunroom to bake dry in properly control, dust-free conditions. During electrophoresis, a negatively charged vehicle is put through a positively charged paint bath and the paint adheres to all the oppositely charged surfaces. And, since it is done at least half-a-dozen times, there is very little chance anything is missed. All undercoat is really good for is to add about 50 pounds of tar-based gunk to the underside of your car, and to lock in any dust or dirt that a quick air cleaning will have soured out.
For more info see How to Choose an Auto Body Collision Repair Specialist