If you’ve been storing paint for a long time, it may contain impurities such as dirt or lumps. There is no need to throw this paint away.
Instead, cut a disk of window screen to fit just inside the paint can. The screen will sink into the paint and carry the lumps to the bottom of the can.
Source: http://www.planitdiy.com/tips-and-tricks/making-old-paint-like-new-again/
