What does fat over lean mean?

The fat over lean rule is where a painting has many different layers that are allowed to dry and others added on top with the layers being thicker. In that case, a bit more oil would be added to each of the upper layers. My technique is to apply the paint very thinly as if airbrushed on, and I work wet on wet using fast drying Winsor and Newton fast drying oil paint as well as Liquin original and white spirit. If your layer has dried and you apply a very thin layer next, I do not feel you should worry. As long as you don’t go too thick and then put a thin coat over the top you shouldn’t need to worry about your oil paint cracking.