Question for the pro's

^^^^^ I did the body work/paint myself like said 4 years ago so I know what I have under the paint.

I agree always best to go down to bare metal for several reasons, like you said looking for bondo, also previous body work, rust.
I have had some cars with the original paint and thatOEM primer as so tough I wore myself out with my DA and 80 grit. I finally just sanded to bare metal where there were obvious dings, epoxy primed, did my filler, epoxy, high fill urethane primer, sealer, topcoat, I remember one car i did and 8 years later it waas still great, issues.
If you look at the pics above you can see the grinder marks in the metal from work. This car was all original paint. I strip the car with paint stripper. The cheapest I can find. Brush in one direction and use a razor scraper to scrape it in 5-10 minutes. There are times the blade last only a couple seconds. It it gets a chip and starts scratching the metal flip it or get a new one. After most of the paint is off I brush a thick coat on and steel wool the panel. This way you are not putting sander marks in the panel and hiding the original work as seen above. Back in the 70's and 80's they didn't have stud guns to pull dent so they filled them after grinding before the paint and assembly line.

I was a painter on an assembly line. I know what we covered up. I never remove paint with a DA or sander. Unless its for a feather edge on a panel repair.

Look closely at the pics above no sander marks except under the paint. I use a wire wheel to clean the edges in some cases when edge damage I use a DA