Use the 6 inch DA sander, it will get in to most areas, rubbing with accuracy, rather than a small disc that will dig out areas, and leave a poor ripple effect to a freshly painted panel
for general sanding, you want 180 grit, use only 120/80 grit around repairs, then finish with 180... small intricate areas should be done by hand, with 180 grit and/or red Scotchbrite,
The above will leave panel ready for 4/5 coats of 2k filler primer...
Thom