Professional paint manufacturers and sellers will use a paint-shaking machine to mix the paint before they sell it. This will ensure that the paint has not settled and separated. Paint that has settled will be impossible to use effectively. If you're going to use old paint, you will have to shake it at home and mix it by hand to make sure that it is mixed properly.
Things You'll Need
Ensure that the lid is tightly sealed by laying a cloth over the lid and hammering it into place tightly. If the lid is loose, the paint will spill out.
Hold the paint with two hands. One hand should be on top of the paint can and the other should be on the bottom.
Hold the paint can upright and then shift hands so that the can is upside down. Apply pressure to make sure that the lid doesn't slip off.
Repeat the process, shifting hands up and down, for several minutes or until your arms get tired.
Open the paint can immediately by hammering a flat-end screwdriver in between the lid and the paint can and prying the lid open.
Stir the paint with a wooden paint stirrer. You may notice some clear liquid in the paint that has separated from the pigment. Stir until the clear liquid is completely mixed into the pigment.
If you don't want to mix your paint at home, take it to a home improvement store and ask them to shake it in their machine.