The “fat over lean” rule allows you to build a painting that is flexible so over time there will be less cracking to your painting.
The under layers of a painting should be leaner than the upper layers. There are two different approaches to building paint layers following the “fat over lean” rule.
The first approach uses the same ratio of painting medium to oil color throughout a painting; however, the fat content of the painting medium is modified between each paint layer. In the initial layers, the oil medium (fat) is mixed with Gamsol (lean). As you add layers, increase the oil content of the medium by adding Galkyd oil painting mediums or a drying oil (Linseed, Stand or Poppy). The bottom layers will have more Gamsol and less oil. The top layers will have more oil and less Gamsol. The ratio of the painting medium mixture to oil colors remains the same.
The second approach uses varying amounts of the same painting medium throughout a painting. Since oil painting mediums are fat, when you add medium to oil color, the oil content increases. In the initial layers of a painting add a minimal amount of painting medium; then increase the amount of painting medium as you build up paint layers. The ratio of medium to paint increase as you continue painting.