I'm no bodyman, so don't take this as gospel. It's part of my collection of knowledge from reading/trying it a few times, and things that 1st timers often miss.
- Bondo is invented to be wasted. Use it over a much larger area than you think you need to fix a flaw. When you sand you'll blend it much nicer.
- Buy a long board, it will make the surfaces look much better than with a small sander/DA.
- Buy some glazing putty and use it, it's great stuff for those final deep scratches that aren't big enough to bondo.
- Before you shoot primer, I'd finish with 320grit on a DA, or 400grit prior to paint.
- For wet sanding, start with 1500 grit, move to 2000 grit, and use cutting compound on a power buffer after that. (then polishing compound, then a protectant/wax)
Bodywork is time consuming, but much more so for the novice when you don't know the basics, it takes a lot longer to achieve mediocrity without those basics.