Question: In respecting the rule in oil to paint fat over lean, which mediums thin out paints and which fatten them? Can you make oil paint too lean or too fat?
The “fat over lean” rule allows you to build a painting that is flexible so over time there will be less cracking to your painting. Another way to think about “fat over lean” is “flexible over less flexible.”
Because of the high pigment load of artist’s grade oil colors, paint out of the tube should be thought of as “lean.” Adding straight solvent to oil colors thins out the oil binder of the paint, thereby weakening its ability to form a strong, permanent paint layer. Thinning with a mixture of solvent and binder (described below) is a better choice, especially in the early stages of a painting. Painting mediums should simply be thought of as “fat” as they increase the flexibility of paint layers. If using a straight drying oil or Solvent-Free Painting Medium to thin oil colors, use these in moderation – up to 25% by volume – with oil colors.
Putting it to Practice
The under layers of a painting should be leaner than the upper layers. There are two different approaches to building paint layers following the “fat over lean” rule.
The first approach uses the same ratio of painting medium to oil color throughout a painting; however, the fat content of the painting medium is modified between each paint layer. In the initial layers, the oil medium (fat) is mixed with Gamsol (lean). As you add layers, increase the oil content of the medium by adding Galkyd oil painting mediums or a drying oil (Linseed, Stand or Poppy). The bottom layers will have more Gamsol and less oil. The top layers will have more oil and less Gamsol. The ratio of the painting medium mixture to oil colors remains the same.
The second approach uses varying amounts of the same painting medium throughout a painting. Since oil painting mediums are fat, when you add medium to oil color, the oil content increases. In the initial layers of a painting add a minimal amount of painting medium; then increase the amount of painting medium as you build up paint layers. The ratio of medium to paint increase as you continue painting.
Scott Gellatly is a landscape painter living and working in Portland, Oregon, and the Product Manager for Gamblin Artists Colors. To learn more about Scott and view his work, visit his website.
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