Understanding the “Fat Over Lean” Rule in Oil Painting
- cheshireartclasses
- Nov 4
- 5 min read
If you’ve ever worked with oil paints, you’ve probably heard the saying:
“Always paint fat over lean.”
It sounds simple, but what does it really mean — and why does it matter so much?
Let’s break down the science, the technique, and how you can use this rule to make your paintings last a lifetime.
What Does “Fat Over Lean” Mean?
In oil painting, “fat” and “lean” refer to how much oil is in your paint mixture.
Fat paint has more oil (from the paint itself or added mediums like linseed, walnut, or stand oil).
Lean paint has less oil, usually thinned with solvents like odourless mineral spirits or turpentine.
The rule “fat over lean” means that each new layer of paint should have slightly more oil than the one beneath it.
Why It Matters: The Science Behind the Rule
Oil paint dries not by evaporation, but through oxidation — a slow chemical reaction that causes the oil to harden.
Lean layers dry faster because they contain less oil.
Fat layers dry slower because they have more oil.
If you put a lean (fast-drying) layer on top of a fat (slow-drying) one, the top layer can dry first and crack as the slower layer underneath continues to move and cure.
By applying fat over lean, each new layer remains flexible and adheres safely to the one below — preventing cracking, delamination, or dulling over time.
How to Apply Fat Over Lean in Practice
Here’s how to follow the rule in real-world painting:
Start Lean (First Layers)
Mix your paint with a bit of solvent (like odourless mineral spirits).
This thins the paint and reduces oil content.
Ideal for underpaintings, sketches, or blocking in large areas.
Example:
1 part paint : 1 part solvent (no medium).
Build Up with Mediums (Middle Layers)
As you add layers, gradually include a bit of medium to increase oil content.
Common medium: 1 part linseed oil + 2 parts solvent.
This adds richness and smooth handling without overloading oil too soon.
Example:
1 part paint : 1 part medium (mix of oil + solvent).
Finish Fat (Final Layers & Glazes)
Your top layers should have the most oil, often pure paint or paint mixed with a bit of stand oil, linseed oil, or glazing medium.
These layers stay flexible and glossy — perfect for highlights and colour depth.
Example:
Paint straight from the tube, or with a small amount of stand oil for extra gloss.
A Simple Ratio Guide
Layer | Medium Type | Fat Level | Drying Speed |
Underpainting | Paint + solvent | Lean | Fast |
Mid-layer | Paint + solvent + small oil | Moderate | Medium |
Final layer | Paint + oil (little or no solvent) | Fat | Slow |
⚠️ Common Mistakes to Avoid
Too much oil too soon: Makes the painting tacky or prone to wrinkling.
Skipping fat over lean entirely: Leads to cracking and poor adhesion.
Inconsistent medium ratios: Try to increase oil gradually — not suddenly.
Using fast-drying mediums on top: Always apply them lower in your layers - acts like Lean fast drying layers.
🌟 Using Mediums Intentionally: Control the Fat, Flow, and Finish
Mediums aren’t just about thinning paint — they’re how you control drying rate, texture, and sheen across layers. Understanding how each ingredient behaves helps you build a stable, beautiful surface.
1. Linseed Oil – The Classic Workhorse
Properties: Strong binding power, relatively fast-drying.
Use: Ideal for underpainting or early “fat” layers where flexibility and adhesion matter.
Finish: Produces a glossy surface with warm tone enhancement.
Tip: Refined linseed oil yellows less than cold-pressed. Avoid overuse in upper layers if you want an even sheen.
2. Stand Oil – For Smooth, Glossy Depth
Properties: Thickened linseed oil that dries very slowly and levels brushstrokes beautifully.
Use: Perfect for final layers and glazes when you want smooth blending or a glass-like surface.
Finish: High gloss, enamel-like sheen.
Tip: Mix stand oil with a small amount of solvent (like 1 part stand oil to 2–3 parts OMS) for easier handling.
3. Walnut Oil – Smooth and Balanced
Properties: Slower-drying, less yellowing, softer handling feel.
Use: Great for mid-to-final layers where you want subtle flow without tackiness.
Finish: Satin sheen with a slightly softer, more diffused glow than linseed.
Tip: Excellent for painters sensitive to solvents, as walnut oil can be used alone as a medium.
4. Poppyseed Oil – Gentle and Light
Properties: Very slow drying, pale in color, and less prone to yellowing.
Use: Ideal for light colors and top layers, but not suited for lower layers (too slow-drying).
Finish: Soft satin or semi-gloss.
Tip: Use sparingly; avoid placing faster-drying layers (like linseed-based paint) over it.
5. Alkyd Mediums – Modern Speed & Durability
Properties: Synthetic resins that speed drying without breaking the fat-over-lean principle.
Use: Excellent for artists who want faster layer turnover but stable buildup.
Finish: Varies by brand — from satin to gloss.
Tip: A small amount goes a long way; adding too much can cause uneven sheen or surface tackiness.
💫 Managing Sheen Across Layers
A painting’s surface gloss changes naturally as it dries. Early, lean layers often dry matte or sunken, while fatty top layers reflect more light.
Here’s how to keep it balanced:
Use consistent medium ratios across each layer to prevent patchy gloss.
Apply a retouch varnish once dry to even out dull spots before continuing to paint.
For the final surface, choose a final varnish (matte, satin, or gloss) that complements your aesthetic.
Remember: gloss increases depth and saturation, while matte finishes soften edges and reduce glare.
Think of sheen like mood lighting — it sets the atmosphere of your painting. Use it intentionally, not by accident.
🌟 Fat Over Lean — A Rule, Not a Law
It’s worth remembering that “fat over lean” is more of a guide than a commandment. While it’s rooted in solid chemistry, art isn’t chemistry alone — and not every artist follows the rule with precision.
Many contemporary painters, especially alla prima or impasto artists, work in single sessions or blend wet-into-wet — meaning all layers dry together, and fat-over-lean ratios become less critical. Others deliberately experiment with texture and drying patterns for expressive or abstract effects.
The key is understanding the rule so you can break it intelligently. If you know how oil behaves, you can bend the guidelines to suit your style — without accidentally compromising the stability of your work.
So, think of “fat over lean” as a safety net, not a cage. Follow it when you’re layering over days or weeks, but feel free to push boundaries once you know how your materials behave.
Final Thoughts
The fat-over-lean rule remains one of the cornerstones of traditional oil painting, but it’s not meant to stifle creativity. Think of it as a flexible guide for building stable layers that age well — not a rigid formula to fear.
Once you understand the reasoning behind it, you can confidently decide when to follow it and when to break it in service of your vision. That’s where real mastery begins — balancing craft with creative freedom.
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