Fat over lean in oil painting refers to the amount of oil in various painting layers. The ‘fat’ part refers to more oil in the paint and the ‘lean’ part refers to there being less oil in the paint.
When painting in layers, the rule states that it’s important to start with less oil in the mixture by adding a solvent for your first layer(s) and then to stop adding in solvent for subsequent layers.
This is because if a top layer of paint dries before the layer beneath does, cracking will likely occur in your painting.
Oil paints are pigments mixed with oil straight out of the tube and therefor they’re already ‘fat’, when using oil paints we can add even more oil to the mixture by adding something like linseed oil.
We can also add solvents to the mixture such as Gamsol which will break down and reduce the oil content of the mixture, adding solvents will make your paint ‘leaner’.
Please note that if you do not use any solvent or if you paint alla prima (wet on wet), this rule doesn’t apply to you. Further, if you add in mediums to your paint mixture such as Liquin, this rule doesn’t apply either – this rule only applies if you use solvents in your paint mixture.
Most painters like to tone their canvas or painting surface before starting a piece, generally to get rid of the white and to add in a more neutral starting colour so they can get an easier understanding of the true colours of their paint. T
his is typically done by mixing an oil paint with solvents, this will break down the oil content of the paint and makes it semi transparent, sort of like a watercolour appearance.
The painter is then free to proceed to adding in their paints with little to no solvent and this will correctly follow the fat over lean rule.
If you would like to use solvents throughout your painting, just remember to add in less each time you add another layer of paint and you will correctly follow the fat over lean rule, thus avoiding cracks in your painting.