Fat Over Lean in Oil Painting: A Better Model for Layered Indirect Painting

Fat over lean is a foundational guideline for building layered oil and alkyd paintings. It is commonly summarized as “more oil (fat) over less oil (lean).” However, Mayer framed the underlying principle more broadly: a less flexible layer should not overlie a more flexible one (Mayer 1979, 165).

In other words, the rule is really about flexible over brittle. As oil paint cures and ages, it can lose flexibility and become more brittle (Mayer 1979, 165). Meanwhile, oils cure by autoxidation and continue to change after the paint is touch-dry (Pizzimenti 2021, 3). Therefore, if an upper layer becomes comparatively rigid while the layer beneath remains more elastic or is still changing, stresses can accumulate, increasing the likelihood of cracking (Mayer 1979, 165).

This matters most when you paint in distinct layers over time. By contrast, in alla prima (wet-in-wet) painting, the layers are not curing as separate laminates.

However, the rule is also a frequent source of confusion. Artists are often encouraged to choose a “fat” or “lean” medium. That framing can be misleading, because “fat” and “lean” are properties of the dried paint layer, not permanent properties of a painting medium.

A more useful way to think is structural: wet paint behaves like a colloidal dispersion (a sol), while the cured paint film is a solid composite—pigment particles dispersed within a crosslinked binder matrix. Therefore, the most practical studio question is not whether a medium is “fat,” but whether the dried layer is binder-rich or binder-poor relative to its pigment packing.

This article reframes “fat-over-lean” in terms of pigment volume concentration (PVC) and critical pigment volume concentration (CPVC). Meanwhile, it traces where the phrase appears (and does not appear) in major reference texts.