Home depot sells a product , sorry I do not remember its name, to paint onto the ceiling and then repaint it with regular house paint. I did my fathers 20 years ago
Kathey...You need to find out where the leak is coming from and patch or repair.
Depends upon what caused the spot. If it is water, you have another problem far more important.
You can cover the area with Kilz paint or use a clearcoat of some sort, then repaint