Fat over Lean Oil Painting Lesson

Fat over Lean has nothing to do with painting thick paint over thin paint. It actually refers to the amount of oil in the paint. The underlying layers should be mixed with less oil (be leaner) than the upper layers which should be mixed with more oil (fatter).

Each layer should be dry to the touch before another layer is applied to prevent cracking. 

This video by Winsors & Newton does a great job explaining it all.