I've used two different types of undercoating on my wheel wells to repair chipped spots (white paint showing through from rock impacts).
The first was an asphalt based undercoating. I stay completely away from this type now. It is very brownish black color when dried, takes forever to completely dry, and smells like fresh asphalt until thoroughly dry (can be days). It also sprays really "chunky" or splotchy. The lasting smell would probably make it less than ideal for a customer's car.
The second is a rubberized undercoating/sound deadener. The stuff I used was in a blue can with white lettering (from WalMart, AutoZone, etc). It sprays much finer (but still leaving a texture), and doesn't have a lingering odor. It is a matte black, and dries relatively quick (depending on coat thickness). It is extremely durable, too. And it really looks OEM applied. Also great for the parts of the undercarriage that are visible if they were previously undercoated.
Downfall to doing this is it is time consuming. Figure in cleaning the surface, taping surrounding areas, and dry time. I wouldn't do it on a windy day just because of overspray, either. I can't remember how long it took me to do my four wheelwells, but it wasn't a quickie type thing (I had removed wheels to do it once).
But once finished, I think it looks much better than other options because of the totally fresh OEM look. And it is more durable than protectants that will wash/wear off, and paint that will chip. If you want more of a sheen, you can always spray a protectant on it after it's dry, as it is made of rubber.
Just some thoughts.
Dave
