As long as they're not the super cheap, plastic over fiberglass panels, most tiles will take normal house paint. I've custom painted lots of them. Ideally you'll want to lay them out in a place where you can use a sprayer. I use a good ol' Wagner power painter and it works great. If I'm going to paint the grid work I hang all the metal parts from the shop ceiling and spray them with the appropriate paint before installing. As for the stained ones, use Kilz primer first. It's designed specifically to cover up stains. If the panels are damaged, swollen, etc, just replace them. They are not expensive.
Slim Jim
August 2009