I'm in the process of removing popcorn and repaint the ceiling.
After a few days of filling holes in my ceiling, the ceiling was primed. Staring at a primed ceiling, it becomes apparent that there's a joint that's sticking out.
Is it too late to do skim coat and prime again now?
Absolutely you can skim. (Lots of people deliberately prime in order to see the defects that might not be obvious.)
If you can, use topping mud, which will sand more easily and give a better feathered edge than all purpose mud.
One of the things that happens during home renovations is the DIYer in the house (raising my hand as guilty) is cautious of creating a snowstorm of mud (joint compound) that is sanded off. This leads to trying to use thin layers of mud which means less sanding.
While our intentions are good this leads to tape jobs sticking out, edges not feathered enough, and my personal favorite - is "bubble holes". I describe these as defuncts in the mud that lead to bubbles but during the sanding process. But especially with the smaller ones these holes are often covered by packed loose mud.
What happens? As soon as you paint the sheen and wetness highlights your bad joints, places not feathered enough, and the "bubble holes". You must apply a generous skim coat here, maybe two coats, a sanding in between, and then start the priming process over again. And now you have to feather your primer so the sections already primed aren't "thick".
