When using paint from a previously opened can, it’s best to pass the paint through a filter to screen out dried paint chips and other small bits of debris.
You can buy disposable paint filters, but they’re flimsy, not reusable and the paint flows through much too slowly.
Instead, use a piece of vinyl insect screening. Cut the screen about 12 or 14 inches square, secure it to a new, clean paint can with three small spring clamps, and then pour the old paint through the screen.
When done, rinse out the screen with a garden hose and save for the next paint job.