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‘Fat over lean’ refers to the principle of applying ‘fat’ oil paint, which has a higher oil to pigment ratio, over ‘lean’ oil paint, which has a lower oil to pigment ratio, in order to ensure a stable paint film. The idea is to prevent upper layers of oil paint from drying faster than lower layers, which can lead to an oil painting cracking.
Oil paint straight out of the tube is considered ‘fat’. Adding more oil, such as linseed oil, will make it even ‘fatter’, further increasing the drying time. Even when it feels dry to the touch, it may still be drying under the surface.
‘Lean’ paint, on the other hand, is oil paint mixed with turpentine or some other fast-drying medium. ‘Lean’ oil paint dries faster than ‘fat’ oil paint.
The key thing to remember, in order to avoid cracking, is that every layer in an oil painting must be ‘fatter’ than the one beneath it. That is because if ‘lean’ oil paint is painted over ‘fat’ oil paint, the ‘lean’ layer could be subject to cracking as the ‘fat’ layer dries and contracts underneath it. Also, lower layers tend to absorb oil from the layers above them, especially when ‘fat over lean’ is not followed.
Another thing to consider is the quality of the oil paints you are using. Cheaper oil paints often have drying agents added, making the drying times more consistent.
Conversely, quality oil paints generally consist of oil and pigment only, leading to varying drying times. For example, Prussian blue, titanium white, and flake white all have a lower oil content and dry more quickly. However, cadmium yellow and cadmium red, both of which have a medium oil content, usually take about five days to dry.
According to some oil paint manufacturers, it is possible to circumvent the ‘fat over lean’ principle by using synthetic, alkyd-based mediums like Galkyd and Liquin. However, while these products do provide consistent drying times, increase the paint film flexibility, and promote adhesion between paint layers, their long-term stability is not known at this time.
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