Fat Over Lean Rule in Oil Painting

George O’Hanlon wrote an article for Natural Pigments titled Fat Over Lean: Essential Guide to Oil Painting Techniques for Artists (11 August 2024).

« The fat over lean rule in oil painting refers to the practice of applying paint with a higher oil content (fat) over paint with a lower oil content (lean). The underlying science behind this rule is rooted in the drying process of oil paints. Lean layers, which have less oil, dry faster and are less flexible, while fat layers, with more oil, dry slower and remain to some degree more flexible. This difference in drying times and flexibility is crucial because applying a less flexible layer over a more flexible one can lead to cracking as the painting ages. »

« To effectively apply the fat-over-lean rule, consider the following steps:

Start with Lean Paint: In the initial layers, use paint straight from the tube or thinned with a solvent. This ensures a lower oil content.

Increase Oil Content Gradually: As you build up layers, gradually add more oil or a medium containing oil to your paint. Each layer should be slightly “fatter” than the one beneath it.

Avoid Over-Thinning: Avoid over-thinning the top layers with solvents, as this would reduce the oil content, thereby increasing the pigment volume concentration and promoting faster drying, violating the fat-over-lean rule.

Think in Terms of Pigment Volume Concentration (PVC): This concept refers to the ratio of pigment to binder (oil) in the dried paint. At the critical pigment volume concentration (cPVC), there is just enough oil to bind the pigment without leaving excess oil in the paint film. Paint at this stage is flexible, semi-glossy, and less prone to issues like sinking in or cracking. »

« Allow each layer to dry thoroughly before applying the next. This prevents issues like alligatoring, where the top layer cracks as it dries faster than the underlying layer. »

« Is it okay to mix different types of mediums, like walnut alkyd and Venetian Medium, in the same painting?

Yes, you can mix different mediums. However, it’s important to remember that alkyd mediums generally dry faster and should be applied beneath slower-drying mediums like Venetian Medium. This approach aligns with the fat-over-lean principle since faster-drying layers should be underneath slower-drying ones to avoid cracking. »