Craig:  Mix about 20% (by volume) of micro balloons into your WLS sandable primer paint to make it even more filling of light scratches and small holes.  Further, apply two or three coats of this "extended" primer at 15 minute intervals to permit the solvents to dry somewhat so that the paint stays where you spray it.  You will have to use a larger orifice on the spray head but this technique should save you time because you can prime and sand just once or twice instead of 20 times as I did before I learned this trick.    Greg Nelson