100 grit rule

Everyone seems to have different preferences, but depending on what you're doing starting with really rough paint or rust/bondo 80,120,220,320 worked for me.
That was good enough for "general production" paint. To make the paint look better with no chance of sand scratches 400 was needed.

Using a DA in the early stages correctly(flat on the surface) is a time saver...too many people try to save more time by using the edges and getting DA waves?

Block sanding with short blocks and Long block is critical and Making your own for curved areas is helpful