||even if you're not doing your Z in a dark color, you still want it straight. Sanding through a just rock chip will still leave a serious ripple in any final finish, not to mention door dings and other small dents that were repaired. The purpose of a high-build polyurethane catalyzed primer is to give you enough depth to be able to get a flat surface. Epoxy is a fine product to promote adhesion and prevent rust on bare metal, but is not intended to be sanded.
There are flexible foam sanding pads, but I often use even a wood stir stick with a 12" piece of 220 dry. When I think it's straight, I'll prime it again and look to see how straight the panel looks while wet. It usually requires another coat of primer. Next I use 320 dry then 400 wet. If you apply enough paint (usually basecoat), the 400 grit scratches will be filled. The reason for the catalyzed primer (with all solvents dried over night) is to prevent the primer from sinking into the body filler scratches underneath.
The nice part about base/clear coat is being able to deal with any imperfections before you apply the clear. It also protects the color coat from UV's and weather. You also can't wet sand then polish a single-stage metallic finish. Hopefully, the clear goes on "wet" enough to just sand with some 1000, then 2000 and then polish to a factory look.
I know I haven't posted for awhile, I was just concerned that I didn't see another Z wave at me when it drove by. Good luck.
PS I just painted 2 totalled CTS's and the second floor where one of my renters had a hemp farm for the past 4 years. Besides that, I'm into fast boats. Life is good.