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 same rule.

Slow drying layer is more flexible relative to a fast drying one.

Fat layer dries slowly, thus it is more flexible.

Thick layer dries slowly, thus it is more flexible.

By now you can probably see the trend - it all comes down to flexibility!

Can we break the rule?

To be clear, I am not saying that we can or should break the rule. I just disagree with it's name. It should probably be called something like "more flexible over less flexible" instead. That said, here are some interesting examples to consider:

Traditional glazing technique

In this case multiple thin glazing layers are applied over thicker underpainting layer. This is perfectly fine because the underpainting is typically made with tempera or other fast drying paint and the glazing layers are more flexible. Even if the underpainting is done with oils, it must be completely dry before proceeding with the glazing so the thickness doesn't really matter. Also, the glazes are made of little paint and lots of medium which makes them fat compared to the underpainting.

Thin layers of oil paint over thicker layers of alkyd paint

This is acceptable for the same reasons as the glazing technique. The alkyd paints are fast drying so it might be OK to use thick layers of them under thinner layers of oil paint.

Thin layers of slow drying paint over thicker layers of fast drying paint

This is just another variant of the same thing. Even if you use the same paints from start through finish, you can still "break" the rule if you know how fast each paint is drying. For example, you can have a thick layer of Prussian blue, cobalt or other fast drying color and put a thinner layer on top of it that dries slower. This is off course very tricky and does require experience with the particular paints. You should be very careful and it is best to avoid it if possible.

Alla Prima

In alla prima the entire painting is completed in one session. This means that there are no well defined layers. Everything is applied "wet in wet". The layers are often mixed within each other on the canvas. Even if you put layers on top of other layers without mixing them, they are both "wet" and this makes them form very strong connection (via polymerization) to each other. The painter should still follow the rules and use thick over thin or fat over lean, but with alla prima this is needed more due to the behavior of the oil paints and less to prevent cracking. Also, it is already difficult to break the rule in alla prima simply because placing thinner or leaner layers over thicker or fattier is typically difficult and does not produce appealing results anyway.

Conclusion

The fat over lean rule is VERY important and we must follow it as much as possible. However, it would be better to think of it as "more flexible over less flexible". That would give us more freedom and would help us understand the drying process better.