Everything you need to know about the fat over lean rule
This is probably the most important technical rule in painting. As such, it is very popular but probably not very well understood. As any rule, if you don't fully understand it it is best to follow it just in case. However, it is even better to understand it and then you would be able to decide if and when to follow it blindly and when and how you could afford to work around it. The primary reason for the existence of this rule is to help prevent painting layers from cracking. Even though the reasons for cracking are often very complicated, they can ultimately be simplified to one single condition: If a non-flexible layer lies on top of flexible one, the later will shrink and cause the top layer to crack. That said, the fat over lean rule can actually be formulated in several different ways: Fat over lean Slow drying over fast drying Thick over thin Flexible over less flexible If you paint with oils in multiple layers, then these are all different variants of the sam...