The Three Oil Painting Rules: Fat over Lean, Thick over Thin, and Slow over Fast Drying

The "Three Oil Painting Rules" (I imagine this being said with a big booming voice) are the primary rules to help your painting last through the centuries without cracking. They are: fat over lean, thick over thin, and slow over fast drying. 

Fat Over Lean

Simply put, each layer needs to have more oil than the previous one, making each layer fattier in order to avoid cracking. The reason for this is that even if paint is dry to the touch, it normally takes six months to a year for a layer to be completely dry. 

During the drying time, each layer moves and settles as it hardens. If the base layer is fattier than its upper layers, the upper layers will dry faster and they will harden while the under layer is still moving. This causes cracking. 

In this diagram from Gamblin, you can see how the different layers are added upon. Remember, Gamsol is their artist solvent. 

Painting Layers with Solvents:                             Painting Layers without Solvents: 

Leanest = Paint + Solvent                                     Leanest = Paint + Alkyd 

Lean = Paint                                                          Lean = Paint

Fat = Paint + Solvent + Oil                                   Fat = Paint + Alkyd + Oil 

Fattiest = Paint + Oil                                             Fattiest = Paint + Oil  

The Fat over Lean rule is the most important of the three. How you do it isn't nearly as important as making sure you are doing it. Experiment with mixtures and mediums, just remember add more oil. 

Pre-mixed mediums, such as galkyds, alkyds, and liquins are the opposite. You will use more medium in the beginning to speed drying time and less and less as the painting progresses. 

Thick over Thin 

Add slightly thicker layers of paint as you progress in order to avoid cracking. This is usually a natural progression throughout the painting and is not something you have to be super cognizant of. 

Note: Glazing is an exception to this rule. 

Slow Drying over Fast Drying

Normally you mix colors together and this rule can be forgotten. However, if you are glazing a fast-drying paint over the area of a slow-drying paint, you better be adding a lot of oil in order to avoid cracking. 

Below is a general color list of Slow and Fast drying oils: 

Fast Drying - around 2 days 

Cobalt Blue, Prussian Blue, Flake White, and most brown earth tones such a Raw Sienna, Umbers, Vandyke Browns, etc. 

Medium Drying - around 5 days

Phthalo Blues and Greens, Burnt Sienna, Cobalt Violets and Green, Ultramarine Blue, Mars Black (all Mars Colors), Sap Green, Permanent Alizarin Crimson, Ochres, Cadmiums, Titanium White, Zinc White, Lamp Black, Ivory Black, etc. 

As you can see, just about all of your colors fall into this category. Meaning figuring out what colors need to be in which layer is usually unnecessary. 

Slow Drying - more than 5 days 

Some Yellow and Oranges (excluding all cadmiums as they are medium drying), all Quinacridones (so slow... so slow....), Alizarin Crimson, etc.

For more information about drying times of specific paints read here for Winsor and Newton or here for M. Graham.  

Below is a video from Winsor and Newton that illustrates, albeit kind of in a tacky way, how cracking can occur: 

To read more about the three rules, especially fat over lean, read here, here or here. For more about painting check out my main Tips and Techniques page.