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.