When working with bondo...i would sand the bondo down with a disc sander and then block sand it with 80 grit. There would always be valleys or low spots or air bubbles leaving pits etc.
I would just go over it with a thin layer of bondo and sand... and repeat the process until i got all the pits valleys or out. Then still using a rubber block i would wet sanded it with 300 grit until smooth.

Then feather out the sanding marks of on the original paint in the area worked upon and hit it with a couple coats of primer which would fill in those tiny pin holes. Then painted it.

Primer and paint were all from spray can but i had that bondo so smooth that it that it worked out fine.

Starrider