FMX, If you are going to guidecoat and block out a catalyzed primer, you can use 150-180 grit for prep. It beats the hell out of sanding for days to get it ready. I have no reservations about priming over 150 grit scratches. I just add an extra coat or two to make sure they will be filled. I usually spray 3-4 coats, depending on how much bodywork there is. On problem spots you can even add 2-3 more. I use 360 to block the primer, and by the time you have a couple coats of base on there, there is no sign of scratches. I guess after 40 years of painting, I don't want to sand any more hours than necessary! :-)