On long storage, the solid constituents of paints, notably the pigment, tend to settle out.

Unless the settled material is properly is reincorporated, the opacity and other properties of the paint may be impaired. thorough mising witha broad blade stirrer, using a lifting and beating action, is effective for small quantities of paint.

For large quantities, a mechanical shaker may be more practical.  When paints, known to be prone to settlement, are stored inverting the containers at regular intervals is helpful.