Fat over lean explained: oil paint drying times and how to layer oils

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If you only learn one technical rule in oil painting, make it fat over lean. Ignore it and a painting can crack, wrinkle or peel months later, long after it looked finished. The rule sounds mysterious, but it comes down to how oil paint dries.

How oil paint dries

Oil paint does not dry by evaporation, as watercolour or acrylic do. The oil in it, usually linseed, reacts with oxygen from the air and slowly hardens into a solid film. That process carries on long after the surface feels dry to the touch, and the film changes slightly in size and flexibility as it goes. That is why there are several stages people talk about:

Touch dry: you can touch the surface without paint coming off. This can be anywhere from a day or so to well over a week, depending on the colour, thickness and room.
Dry enough to paint over: firm enough that a new layer will not churn up the one below.
Fully cured: the whole film has hardened. For thick paint this can take many months or longer.

What fat and lean mean

Fat paint has more oil in it. It is more flexible and dries more slowly.
Lean paint has less oil, often because it has been thinned with solvent. It is less flexible and dries faster.

The rule is that each layer should be at least as flexible as the one beneath it, so every new layer carries the same or a little more oil (or oil-based medium) than the one before. If a stiff, lean layer sits on top of a slow-drying, flexible fat one, the layer underneath keeps moving as it cures and the top one cracks.

Related rules that go with it

Thick over thin: thin layers first, heavier paint later.
Slow drying over fast drying: a slow-drying layer underneath can keep moving while a fast-drying layer on top has already set, and that can cause cracking.

How to layer a painting in practice

Underpainting: thin paint, with solvent or a solvent-free thinning medium, in one or two colours. It dries quickly and sets out the drawing and darks.
Blocking in: paint used close to how it comes from the tube, perhaps with a touch of medium. Big shapes and colours go in.
Development: if you add medium, keep the oil content the same or slightly higher than in the layer before.
Final layers and highlights: the "fattest" paint, which is often simply tube paint, sometimes with a little extra oil or medium.

The common mistake is the reverse: starting with rich, oily paint and then thinning later layers with solvent to make them easier to brush. Keep solvent for the early stages.

Does it matter if you paint in one go?

If you paint alla prima, wet into wet in a single session, the layers mix together on the canvas and fat over lean matters much less. It becomes important once you let layers dry and return to them.

Things that change drying time

Colour: some pigments speed up drying and some slow it down. You will notice that certain colours on your palette stay wet much longer than others.
Thickness: thick paint and impasto take far longer to dry through.
Mediums: some mediums are designed to speed drying, others to slow it.
Room: warmth, light and moving air help; cold, damp, dark rooms slow everything down.

A simple routine

Keep notes of what medium you used in each layer.
If unsure, press gently on a thick area: if it is soft underneath, wait.
When in doubt, do not add more oil to an early layer; save it for later ones.

Have you had a painting crack or stay tacky for weeks? Describe your layers in the comments and others can help work out why.