[JCoop]

Believe it or not, the most vulnerable are the tops (underside) of the fenders. The quarter panels don't get them at all. Line the tops of your fenders with 12" X 24" Dynamat, then spray the edges and surrounding areas with rubberized undercoating spray.
The drag radials pick up the stones and spit them out at about 11 o'oclock, right into the tops. a large enough stone and enough speed will create star cracks that are impossible to rub out because they start from the base of the paint layers. I got one on my old small block SPF and coudn't really fix it so I just put fender stripes to hide it.