For anyone reading this that may not be familiar with the Fat-Over-Lean principle in oil painting:
The “fat over lean” principle allows you to build a painting that is flexible so over time there will be less cracking to your painting.
First it is important to understand that fat and lean do not refer to thickness of paint, but instead to amount of oil: paint with more oil content (Fat) over paint with less oil content (Lean).
Painting Fat: If you add oil to the paint out of the tube, the paint becomes fatter. Fat painting is characterized by oil-laden, glossy, and slightly stickier texture, and has much more sheen to it.
Painting Lean: If you add solvent (like Spike Oil or Turpentine) to the paint out of the tube, the paint paste becomes diluted with a drying agent, and thus leaner. The paint will look thinner with less sheen, and have a more matte or satin appearance.
The under layers of a painting should be leaner than the upper layers. There are two common 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 (fat) medium is mixed with a solvent (lean). As you add layers, increase the oil content of the medium by adding a drying oil (Linseed, Stand or Poppy). The bottom layers will have more solvent and less oil. The top layers will have more oil and less solvent. 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.
Hope that helps!