A person used to be able to get an answer for this on Youtube but you probably can't anymore. And while searching you have to listen to 2 ads on every attempt to fnd what you wanted.
Well, I bought a junkyard fender (steel) with base coat/clear coat finish. How to properly sand it for priming to have someone paint match it? What grits to use? Simple questions, I thought.
I don't have a new OEM fender. I don't have to do any filler work. I don't have it already filled and now I'm blocking it. And the other videos available you can't hear the person because he's working in a cavernous shop with other people using air tools all around him in a 23 minute video about all the above.
I just want to sand and prep it myself or someone else to paint. Oh, yeah, FOR THE LOVE OF GOD!
It's been a long time since I did any bodywork, but when I did it was wet sanding with 400 grit sandpaper for paint prep. Sand the area with the 400 and check afterwards to be sure any orange peel is gone. If you need to do repairs and put primer on then use 220 grit for primer, after primer use spot putty for any minor imperfections then use the 400 for paint prep. Clean the panel with a grease and wax remover before doing anything.
Well I got a little better results by searching youtube for "Painting over paint". Evidently you're supposed to scuff the existing paint with a red Scotchbrite type pad for automotive paint prep and a red one is in the 320 grit range. The grey is more like 600 grit I believe. After scuffing you want to degrease/dewax with a remover and at least a blue shop towel, then you're ready for some epoxy primer.
My replacement fender is white and going on an Antique Bronze car, so I may rattle can the underhood flange and door jamb area then take it to the shop for final paint and blending, which they probably won't appreciate. Bu it's at best a 2500 daily driver.