Fat Over Lean (oil painting)

Fat over lean means that each new layer of oil paint should contain more oil (fat) and less solvent (lean) than the layer underneath. This keeps the painting flexible and helps prevent cracking as the paint dries.

1. Fat over lean is a fundamental rule in oil painting that describes the correct order of applying paint layers. The lower layers should contain less oil and more solvent, while upper layers should contain progressively more oil.

   The principle works because oil paint dries through oxidation: layers rich in oil remain flexible for longer, while leaner layers dry faster and become more rigid. If a flexible, oily layer is placed underneath a faster-drying rigid layer, the surface can crack as the lower layer continues to move.

   A typical layered approach is:

   • First layers (lean): diluted with a solvent such as mineral spirits or turpentine, containing little additional oil.
   • Middle layers: slightly increased oil content, often using a medium with a small amount of oil.
   • Final layers (fat): richer in oil, using oil mediums or less diluted paint to create depth, luminosity, and flexibility.

Fat over lean is mainly associated with traditional oil painting techniques and is especially important in paintings created with multiple layers, such as classical glazing methods used by Renaissance painters.

The rule is closely related to other oil painting principles such as thick over thin and slow-drying colors over fast-drying colors. While modern oil paints and mediums can reduce some risks, following this principle remains a standard practice for creating durable paintings.

• Flexibility: upper layers should remain more flexible than lower layers.
• Drying stability: slower-drying, oil-rich layers belong on top of faster-drying layers.
• Layer control: artists adjust the ratio of oil and solvent throughout the painting process.
• Long-term durability: respecting the principle helps reduce cracking and structural damage.