Fat Over Lean: Why the Order of the Layers Decides Whether It Cracks

Adding SOLVENT — turpentine or white spirit — makes paint leaner, because the solvent leaves and the remaining film has less binder relative to its pigment. Adding OIL or an oil-rich medium makes it fatter. So does using a pigment that needs a lot of oil to wet it, which is why some colours are inherently fatter than others straight from the tube. Which means the practical rule is about the sequence of your mediums: start with paint thinned by solvent, and increase the proportion of oil in each successive layer. The traditional phrasing is 'each layer slightly fatter than the last', and slightly is the operative word — large jumps have their own problems.

A drying oil film does not dry by losing something. It GAINS oxygen and cross-links, which initially makes it swell slightly and then contract as the network tightens — a process that runs for years. A lean upper layer completes that process quickly and then has no give left. The fat layer beneath is still contracting and still moving. The brittle skin above cannot follow it, so it cracks — and because the cracks follow the stress they usually appear as a wide net over the whole surface. That is distinguishable from other cracking. Drying cracks from fat-over-lean violations are wide, open, and appear within months to a few years. Age cracks are fine, follow the support's movement, and take decades.

The same logic gives two more rules that are often confused with the first. THICK over THIN: a thick layer takes far longer to dry through, so a thick layer underneath will still be moving long after a thin one on top has set. SLOW over FAST: pigments dry at very different rates. Umbers and lead white dry in a day or two; ivory black, alizarin and titanium white can take a week or more. A fast drier over a slow one is the same trap as lean over fat. So a fast-drying underpainting in earth colours is not just tradition — it is choosing the pigments whose drying behaviour suits being underneath. The traditional palette encodes a great deal of this.

Touch-dry is not dry. An oil layer is touch-dry in days and still moving for months, so the meaningful question is whether it has moved enough that the next layer will not be stretched. Paint a test strip alongside the work with the same paints and mediums, and keep it. A strip that has been through a year tells you far more about your materials than any rule of thumb, and it costs a few centimetres of canvas — which is the test-tile habit from the glaze and print batches. Where a painting must be finished quickly, use a medium designed for it or change medium entirely. Acrylic underpainting with oil over it is a legitimate modern answer; oil under acrylic is not, for the reason the ground rung gives.