A student lays a rich, oily underpainting and then glazes over it with paint thinned only with turpentine. Within a year wide cracks have opened across the glaze. Why does the rule of fat over lean predict exactly this?

B. Lean top layer dried faster over a shifting base, splitting the rigid skin as it shrank

The leaner layer sets first and ends up brittle, while the fat layer beneath goes on drying and shrinking for months. The skin cannot follow that movement and tears, which is what traction cracks are. Reversing the order gives a flexible top that follows whatever the hard layer beneath does.