If I have paint that is stuck on the bottom, but probably can mix it with a mixer, will it be okay? This is primer, just to prime several small areas.
I hate it when I sit on wet paint. The mixer will tickle a bit.
Seriously, what kind of paint and how old? Was it opened or new and stored? What were the storage conditions? Even with all of those details, the answer will be a best guess. Your best bet is to try to mix it up, spread a little and see what happens.
Once paint or primer reaches that state, it's time to go shopping!
I wouldn't consider using it.
I'd tell my customers the same thing!