What Is the Fat Over Lean Rule in Oil Painting?
- James Otto Allen
- 3 days ago
- 3 min read
If you’ve ever looked into fat over lean oil painting techniques, you’ve probably come across the phrase ‘fat over lean.’ It’s one of those traditional rules that gets repeated often - but not always clearly explained.
So, what does it actually mean? And why should you care?
What Does “Fat Over Lean” in oil painting Mean?
In oil painting, “fat” refers to oilier paint, and “lean” means paint with less oil (or more solvent - turpentine is the most common solvent)
The fat over lean in oil painting rule means that each successive layer of paint should contain slightly more oil than the one beneath it.
This ensures that your painting dries properly, without cracking or separating over time.
Why This Rule Exists
Oil paint dries by oxidation, not evaporation. That means the oil reacts with oxygen and hardens slowly over time.
If a lean layer (which dries faster) is painted over with a fat layer (which dries slower), everything stays flexible and stable.
But if you reverse that - putting a lean, fast-drying layer over a fat, slow-drying one - the top layer can dry and harden while the layer beneath it is still moving and shifting.
That’s when you get cracking, wrinkling, or even flaking years later. I write to you here from experience, flaking can easily happen in a few years if paintings aren't stored properly and break this 'fat over lean' rule.
How to Apply Fat Over Lean in Practice
Here’s how I think of it in the studio:
First layer (lean): Paint thinned with solvent or medium containing no extra oil. Great for initial block-ins and underpaintings. I usually add sansodor to thin my oil in the first sketchy layer.
Middle layers: Gradually reduce the amount of solvent and add a little more oil or medium. I tend to stay away from any kind of medium at this stage using earth colours (raw umber, burnt umber, the siennas, the ochres). I've painted quite a few paintings mixing each block of mid tone colour quite thickly then allowing quite a few days for this oil paint only thick layer to dry.
Final layers (fat): The final layer now can have some oil mixed into it, since it should then be left to dry for around a year they say, to completely 100% dry. Linseed oil takes a painfully long time to dry in my experience.
Common Mistakes Beginners Make
Ignoring drying times: Painting a new layer too soon can trap wet paint underneath.
Using the same medium for every layer: It’s better to adjust slightly as you go, or just leave any oily medium to the final layer.
Adding too much oil too early: This can make the surface slippery and cause problems later.
The Takeaway
To sum up:
Lean = less oil (faster drying)
Fat = more oil (slower drying)
Always paint fat over lean so the upper layers dry slower and stay flexible.
Think of it as building a stable foundation - each layer supports the next, both visually and structurally.
Learn the Whole Process
In my upcoming oil painting course, I’ll walk you through:
How to mix and apply mediums safely and effectively
How to structure a painting in layers - from the first block-in to the final glaze
How to control drying times, sheen, and surface texture
How to avoid cracking, sinking-in, and other beginner pitfalls
If you’d like to be the first to know when the course launches (and get early access to studio tips and behind-the-scenes lessons), join my mailing list under this post.
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