Don't know if my method is technically correct, but I use my trusty DA with 80 grit, scuffing up the surface where the paint is still ok, and going to bare metal anywhere that the old paint is questionable. I'm old, so I do a panel or two at a time, then prime with epoxy primer. Once I get the entire car done and rust repair patches, mods, or whatever else then I get serious on the dingin' and pingin', do whatever filler work is required, then touch up the epoxy primer whereever I sanded through doing repairs. Next I DA the car again, this time with 180 and lay on the 2k hi build and get serious on the blocking. 2k is easy to shoot, just use a 2.0 or 2.3 tip and reapply as required while blocking to get the surface right. My final block is with 400 dry, then once more around the car with 2K and start the wet sanding down to 400, then primer sealer tinted to the correct shade for the paint.