A painter at the shop in Tulsa told me to quit sanding at 320 on primer and my comeback rate dropped
I was blocking out a fender last month and the painter looked over my shoulder and said I was polishing the primer instead of giving it teeth. He told me to stop at 320 dry and use a red scuff pad on the edges, and to wipe with a waterborne wax and grease remover instead of the solvent stuff I had been using for 12 years. I tried it on the next three jobs and my base coat lay down way flatter, no mottling around the wheel arches. One car that came back twice for peeling clear last year has been fine for 5 months now. I still hit 600 on the blend panels so I don't see scratches, but the primer gets 320 every time. What grit are you all finishing primer with, and does it change if you're spraying waterborne vs solvent base?
My buddy Marcus over in Broken Arrow ran into this same thing about two years back on a Ram he was doing in white. He kept getting these little shadow lines showing through the base around the door handles, and the painter there told him the same deal, that 600 on primer was just burnishing it and the base had nothing to bite into. Marcus switched to 320 dry on the panels and hit the hard edges with a gray pad instead of red since he was spraying solvent base at the time, and the white laid down clean with no halo. He tried the red pad once on a waterborne job and it was too coarse for the sealer he was using, left scratch marks that showed up in the metallic. So for him it came down to what was going over it, not just the grit number. I'd say your 320 on primer and 600 on blends is right where it needs to be.