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'Fat over lean' refers to the principle, in oil painting, of applying paint with a higher oil to pigment ratio ('fat') over paint with a lower oil to pigment ratio ('lean') to ensure a stable paint film.
Oil paint dries at different rates due to the differing siccative properties of the constituent pigment. However, everything else being equal, the higher the oil to pigment ratio, the longer the oil binder will take to oxidize, and the more flexible the paint film will be. Conversely, the lower the oil content, the faster the paint dries, and the more brittle it will be. Ignoring this practice, even in some alla prima painting, may result in a cracked and less durable paint film.
It has been claimed by some paint manufacturers that the 'fat-over-lean' principle can be circumvented by using synthetic, alkyd-based painting media such as Galkyd and Liquin. These media do provide consistent drying times, increase the paint film flexibility, and promote adhesion between paint layers. However, since classical painting media, turpentine, natural resins, and certain plant oils (linseed, walnut and poppy), applied properly have lasted for centuries, some artists choose to avoid synthetic media since their long-term stability is unknown.
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