painting over old drywall
Usually popcorn has a very weak powdery mud that it is made of.
Sometimes painting right over that will fail because the paint is on a dusty surface.
I usually prime after removing popcorn with a thin clear primer called Gardz that will penetrate and seal that. Then very often requires a full skim of taping compound to make smooth, sand, reprime and then paint.
"It's better to come here with questions before you screw up than to come here after and ask how to fix them."- JS