is a lot of confusion over what "fat over lean" means and how this rule applies in oil painting. It has to do with the relative oil content, and drying rate, of the different pigments.
First, consider how oil paint dries. Unlike water media paints like watercolors and acrylics that dry thru evaporation of the water, oils dry thru a complicated chemical process that involves oxidation, or polymerization. The paint actually absorbs oxygen and expands at a certain point during the drying phase. Imagine then if a thin, dry layer of paint sits on top of a layer that is moving and expanding - the result is cracking and lifting of the top layer. Also, 'fat' paint dries to a more smooth, glossy finish, while 'lean' paint has a rougher more absorbent surface, more suitable for subsequent layers to attach themselves to. This is why we hear "be sure to paint "fat" over "lean" to avoid cracking.
Different pigments absorb varying amounts of oil to reach optimum consistency. A "fat" paint is one that has a high oil content, a "lean" color has less oil. The idea is that a color that is high in oil will dry slower so it is not recommended to place it under one that is lean, or a faster dryer. So which paints are fat and which are lean? Well, usually the transparent colors contain more oil while the opaque ones have less. Unfortunately some transparent colors, like Prussian blue, are high in oil (fat) but are rapid driers while some opaque colors, like Yellow Ochre, have less oil but are slow driers nonetheless. You can usually tell which paints have more oil content as they have a more shinny surface when dry.
When a color has formed a 'skin' and is dry to the touch but soft underneath, this is when it is absorbing the most oxygen and expanding. You never want to paint over this layer.
The most important concern is not whether the paint is fat or lean but whether it is a fast or slow drier. The best way the learn how to apply this rule is to do a simple drying test with your colors. Paint out a swatch of every color and make sure they are the same thickness for every color, and check them all periodically over a few days and note which ones dry faster and which slower. Give them a ranking of: 1. Fast dryer (a day or two) 2. Average dryer (about a week) 3. Slow dryer (a couple of weeks) 4. Very slow dryer (three or more weeks). Use this chart as a reference when you paint.
Here is an excellent exercise that I recommend to help you familiarize yourself with the various characteristics of the different pigments in oils. Paint a black line on a primed canvas or panel and paint out a swatch of each color. In this example you can see I have painted out 5 versions of Burnt Sienna from 5 different manufacturers. I like to get several browns from different companies because they all have unique color qualities. Paint them out from thick to thin and label them with the color, the manufacturer, and the date. The black line underneath helps you see which ones are more transparent or opaque. Then, check them a couple of times a day over the next couple of weeks. Note how fast they dry and when they form a skin - mark this info down under the color. Keep this in your studio as a reference and add every new color you get to the chart.
Here are a couple more recommendations when you are painting in oils to avoid cracking from slower drying layers in the underpainting:
1. Paint in thin layers in the underpainting and add extra turpentine to the paint or to the medium to help it dry faster.
2. If painting in multiple layers it is safest just to wait for the underpainting to dry. Many artists will work on several paintings at a time over a period of weeks.
Also, check out my other two blogs on permanent painting techniques for oils: "This is Why Your Oil Paintings Suck" and; "Guidelines for Permanent Painting in Oils".