As Alvin posted, POR is tough.
That is what I have under the fenders of my trucks.
However, if you have a lot of gravel roads (and/or gravel that makes its way onto the roads), an additional layer of a rubber compound may add a greater degree of protection.
There is not much gravel up here in NY, but I am moving to NC and I hear the gravel when I drive down there. I might be adding a rubber-like undercoat to my Suburban (and to my other trucks that are not yet road-worthy).
