Can paint spoil

Most paints do, either by drying or by rotting. Egg tempera (real egg tempera) can be used for a few days and then it starts to smell bad, caseine paint too, unless lots of preservatives were added. Factory egg tempera and caseine paints contain preservatives to prevent it from rotting.

Oil paint should be kept away from oxygen, because that 's what makes it dry. (if partly dried, it won't make a good paint film anymore). For acrylic paint, it's more or less the same.