What type of undercoat to use to protect the underside of mopar fenders from debris dents. Never understood why mopar did not have an inner wheel well shield for the front fenders to protect it from rock dents.

The inside of the fenders were undercoated for that very reason. The factory undercoating was put on thick and dries hard. It protects the fenders from rock chips. The inside of the 1/4 panels was undercoated as well to help give more rigidity to the panels. RestoRick has a factory undercoating that mimics the factory stuff.

If you aren't concerned with looking factory, I think 3M body shutz can also be used for rock protection as well. Then there is the option of having a truck bed liner coating sprayed on the inside.

Resto Rick, a board sponsor here, sells what you need to do that the right way.