Some beginners and even experienced artists think that “fat over lean” is a painting technique for achieving a particular style of painting. In fact, it means that you are applying the paint so that it does not crack or craze, so your painting will last for many years.
Oil paints dry at varying rates depending how much oil (or fat) is mixed in with the pigment – the higher the oil (fat) content, the longer it takes to dry. Conversely, if you add a medium such as turpentine, it lowers the ratio of oil (fat) to pigment to create a “lean”mixture that dries more quickly.
If you paint a lean mixture over a fat mixture, the lean is going to dry more quickly than the fat. As it dries it contracts, and when the fat layers eventually dry the lean layers will crack. Therefore, if you are going to paint in layers, make sure each successive layer has more fat than the layer beneath. This is especially true if you are painting glazes to achieve a particular effect.