“Fat Over Lean” means to apply a paint layer rich in oil (a fat) over a layer with less oil (a lean) layer on a painting. It does NOT refer to paint thickness.
This simple principle is fundamental to controlling the paint adhesion, drying time and glossiness.
HOW DO YOU PAINT FAT OVER LEAN?
To paint Fat over Lean, use less oil to begin with, and more oil and/or varnish on successive painting days.
1. Lean means non-oily, matte, porous.
2. Fat means oily, sticky, shiny, non-porous.
3. To make a medium or paint leaner, dilute it with a solvent like Lavender Spike Oil Essence.
4. To make a medium or paint fatter, add more Linseed Oil to give it more oil content, or a varnish/balsam like Damar varnish (which will also make it stickier, shinier and dry faster)
WHAT PROBLEMS OCCUR IF YOU DO NOT PAINT FAT OVER LEAN?
Serious archival issues can be caused by not painting Fat over Lean.
When Artists wonder why one layer of paint won’t stick to another, why a fresh layer of paint is ripping off the layer below, why the paint did not dry, dried too matte or too glossy: these are all Fat over Lean issues.
The “Fat over Lean” principle is so fundamental to oil painting that its importance cannot be overstated; yet a basic practical understanding of it by painters is very rare nowadays.
OTHER FACTORS TO CONSIDER
The quality, absorbency and texture of the painting surface is probably the most important element in painting materials as far as determining drying time, adhesion, archival quality and cracking. It has a lot to do with how Fat over Lean will work, as the first layer of paint needs to be fatter than the surface ground in order to adhere properly.
An artist’s understanding of the materials is another important element of the craft of painting. Having better materials is more archival, makes for more options and possibilities, and a greater understanding overall of what one is doing.