I would think too that as course as 320 and you would see scratches, but others here who do good work and I trust have stated they use that grit. I think it has something to do with the fact clear has no pigment and your reapplying clear on top. I have yet to get out and experiment and try it. But with that course, I would think you would need to have a good amount of clear applied, and be very carefull around edges, so you don't break through to base. We are not talking sanding for buffing here sunset, we are talking flattening clear and reclearing. Surely with 320 for buffing you would have a very difficult time stepping down and getting those course of scratches out and also not breaking through or leaving on enough mil thickness.
A finer grit is perfectly fine, but it may take a fairly course grit if the goal is to rid of urethane peel (Moderate orange peel cuts out fairly easily, but urethane peel can prove more difficult).
For normal reclearing of a panel, I normally use 800. Your 600 will be perfectly fine as well. The ones that are going with that course are looking for something more in there paint job, with the goal of being as free of urethane peel as possible. Trying to jog my memory of some of the many things I've picked up from this site, but I think in an old thread somewhere in the archives, Barry K once after stated after doing some test panels, he found around 320-400 gave the best doi when measured with a meter.
Too bad that a few things that have happened over the years have caused many old members to stop visiting, and you don't pick up that kind of information often here anymore. Hopefully some will eventually straggle back in, as talk of things like rustoleum and people who have painted one beater car in there life, stop arguring with veteren painters who are willing to pass on tips for something more then a used car lot repair.
