Paint over Bondo

When you sanded did u strip it all down. Ive seen alot of guys do repair work and when they sand they use edges or the tips of their fingers and u end up w what looks like waves. Blocking everything is your best bet especially if you're a novice
I began the sanding process where the paint was chipping and it didn't take much to get to the Bondo. The lower door area filler is breaking off with the initial sanding, so it looks like I'm going to redo all of it.
Thanks everyone for your input!