My 68 year old paint guy asked why I sand to 600 and I didn't have a good answer
Stopped by my mentor's shop in Amarillo on Friday to borrow his stud welder and he watched me finish a door for about 10 minutes before he said it: "Why you going to 600? Primer don't care past 320." I started to argue and then just stood there because honestly I'd been doing it that way for 12 years without ever asking why. He pulled out a scrap panel, hit half at 320 and half at 600, primed both, and by Monday morning you could not tell me which was which by eye or by finger. His point was that I was burning 3 or 4 extra hours a job on paper that gets buried under primer and sealer anyway. I still think a finer scratch helps on the edges where filler meets bare metal, but now I'm second guessing how much of my routine is just habit I picked up from a guy who picked it up from another guy. What grit do you all actually stop at before epoxy, and did you ever test it or just get taught it?
Man, that scrap panel test only proves what you can SEE and FEEL, not what holds up after five winters of sun and road salt. A finer scratch gives the epoxy more tooth and more surface area to grab, and that shows up years later when a 320 job starts lifting at the edges and a 600 job doesn't. Your mentor proved it doesn't matter for a Monday morning, not that it doesn't matter at all, so I'd keep going to 600 on anything that lives outside.