An Explanation of the Fat Over Lean Rule

The concept I found most confusing when I was learning oil painting was how to follow the Fat Over Lean Rule. I read about it, researched it, and even caused a disagreement by asking a question on a painting forum that left me still wondering, "How do I make sure I'm painting the right way?"

It wasn't until I violated this rule and experienced cracking in my painting that I finally understood! I will share with you what happened and how to better understand this rule, but first, let's talk about paint and mediums.

Sorting Out Paints, Mediums, Solvents, and Oils

In order to understand Fat Over Lean, we need to understand what is fat and what is lean, and why that matters. We're going to get into this further in depth later, but what we want to focus on with fat versus lean is:

• How does a medium affect drying and curing time?
• How does a medium affect the flexibility of the paint film?
• How physically thick are you applying the paint?

Solvents: Lean

First we have solvents such as odorless mineral spirits and turpentine. I don't personally consider solvents  technically to be a medium on their own, but they can be added to a medium. Solvents break up or dissolve the paint and then evaporate. This is why they are frequently used for cleaning up and thinning paint, and especially on beginning layers. Paint with solvent added dries quickly, goes on thin, and creates a weak paint layer compared to plain paint. For these 3 reasons, it's very lean.

Oil Paint from the Tube: In the Middle

Tubed oil paint has a type of oil as the vehicle for the pigment. This means that the powdered pigment colors are mixed together with the same kind of oil you might buy in a bottle to form a paste you know as oil paint! Typically you will find the vehicle listed on the back of the tube, and most often it's linseed oil. This vehicle is what cures over time and forms a paint film, similar to the way tree sap can eventually become amber. It doesn't take as long, though!

Mediums: Fat

Mediums are added to oil paint to adjust its working properties, and they become part of the paint film rather than evaporating. Mediums can come straight out of the tube or be a mixture of oils (shown above), wax, resins, solvents, and more. For simplicity, I like to focus on the use of oils as mediums such as the ones shown here. Just know there's much, much more out there than this!

In general, I like to think of these oils as fats, just like you would consider olive oil fat when cooking. Oils added to the paint may increase or decrease drying time, but curing time continues even after oil paint is dry to the touch. Oils and mediums usually take longer to cure, and create more strength and flexibility in the paint layer than with plain paint. 

The Rule of Fat Over Lean

“Fat Over Lean” - Refers to the idea that each successive layer in a painting should be more flexible than the one before. We want faster drying underneath, slower drying on top. Less oil underneath, more oil on top. Thinner paint underneath, thicker paint on top.

The goal is to have faster drying, less flexible layers on the bottom and slower drying, more flexible layers on the top to prevent cracking or crazing.

To understand the concept of how fat over lean works, imagine a lake that is frozen over but has a moving current underneath the ice. This will cause the ice to crack because it is not flexible and can’t withstand the movement of the water below. 

In oil painting, this would be like following a layer with medium with a layer with plain paint, which is exactly what I did when I learned my own lesson. After a lot of work on a painting using Liquin, I decided to change a large area of the background and used paint straight out of the tube in order to have opacity and cover up the previous layer. At first, everything looked fine! But remember, oil paint takes months to cure and is still moving around like that image of the lake. Six months or so after creating the painting, I noticed a pattern of small cracks over the area I had repainted without medium.

How to Follow the Fat Over Lean Rule 

Above is the correct overall order of layering in oil: First paint + solvent, then plain paint from the tube, then paint + oil or medium. However, you don't have to include all of these layers: You can skip layers or repeat layers, you just don't want to make the mistake of going backwards. 

For example, you could do an entire painting, layer after layer, with just plain paint. No problem!

You could skip from a layer with a bit of mineral spirits in it to a layer with a bit of linseed oil added.

But, once you add oil, you should always add oil. Once you get plain paint, don't start thinning with a solvent.

Mixing Your Own Lean & Fat Mediums

An approach that many people like is to use mixtures of solvent and another medium of choice to control the relative fatness or leanness of their layers. The more solvent you mix in, the leaner it is and the less the fatter the medium becomes.

So your first layer, you could use a mixture of 75% Solvent to 25% Oil as your leanest medium. The next layer might be 50/50, and the next after that could be 25% Solvent to 75% Oil for a fat medium. You could always go fatter by using just the oil mixed with your paint on the final layers.

Looking to Learn the Basics of Oil Painting?

If you're a beginner or you feel you're lacking solid painting skills, check out Foundations of Oil Painting. In this class you'll learn everything from the basics of layering paint to how to control value, how to mix accurate colors, and how to show distance and create strong compositions.

You can access this course by purchasing it, or by signing up for an affordable monthly membership! Check out our membership options here: https://www.schoolofrealistart.com/membership