If a can has a lot of paint out of it than it will form a skin due to the air in the can. Adding some water to latex and thinner to oil will help but paint sometimes have the ingredients start to seperate out and it will dry on the bottom of the can. I have some paint I am storing for a later time and about every 4 months or so I **** the heck out of them. It's good exercise too.
Latex Primer: Good for Drywall, that's about it.
Behr Paint: The plastic buckets are good, that's about it.