A Complete Guide to Oil Painting Mediums and the Fat Over Lean Rule
Oil paint is one of the most versatile painting materials ever created, but artists have always looked for ways to adjust how paint flows, dries and behaves on the canvas. That is where painting mediums come in.
Understanding oil painting mediums can dramatically improve your results. Mediums allow artists to control paint texture, drying time, transparency and surface finish.
My Personal Experience with Oil Painting Mediums
At The Deckle Edge we are often asked about oil painting mediums, and to be honest this was something that confused me for a long time as well.
For years I only ever used turpentine (a traditional solvent used to thin oil paint) because that was what I had been taught. The problem was that the strong fumes often made me feel dizzy, which made painting uncomfortable and after having children it seemed unsafe to have around. When I discovered Gamsol Odourless Mineral Spirits, it completely changed my experience. It gave me the same control when thinning paint but without the heavy solvent smell that turps has.
Another thing that confused me early on was varnishing oil paintings. Firstly, I wasn’t taught thought you had to wait at least six months before varnishing, which is the traditional advice because oil paint needs time to fully cure. Later I learned about Gamvar Picture Varnish, which can be applied once a painting is dry to the touch while still allowing the paint underneath to continue curing. That solved a problem I never knew I had in the first place because that’s how I used varnish previously.
But the biggest confusion for me was all the other oil painting mediums. Terms like Galkyd, stand oil and cold wax sounded interesting, but I did not fully understand how or when they should actually be used. I also found the fat over lean rule confusing and would quietly wonder why I would need to add oil to my paints if they already contained oil. I wanted to properly understand it for myself.
This article is the guide I wish I had when I first started learning about oil painting mediums. My goal is to explain them clearly so that other artists can feel confident using them in their own work.
What Is a Medium in Oil Painting?
In art, a medium is a substance added to paint to change its working properties.
A medium can affect:
• paint flow (how easily paint moves across the surface)
• drying time (how quickly paint becomes dry to the touch)
• transparency (how see-through a paint layer becomes)
• surface finish (glossy or matte)
• brushstroke texture (how visible brush marks appear)
• durability of the paint film (the strength of the dried paint layer)
Word Origin
The word medium comes from the Latin word medius, meaning middle or intermediary.
In painting the medium acts as the material between the pigment and the surface, helping colour move from the brush to the canvas.
Oil paint itself is made from pigment (the coloured powder) mixed with oil (the binder that holds pigment together). Painting mediums are additional materials that artists mix into paint to adjust how it behaves.
A Short History of Oil Painting Mediums
Oil painting developed in Europe during the 15th century, although oil based paints existed earlier. Early painters such as Jan van Eyck, Titian and Rembrandt experimented with mixtures of oils, resins and solvents to control their paint.
Traditional painting mediums included:
Linseed Oil: Oil pressed from flax seeds used to increase gloss and improve paint flow.
Stand Oil: Oil pressed from flax seeds used to increase gloss and improve paint flow.
Natural Resins: Resins such as damar or mastic (hardened tree sap) used to create luminous transparent glazes.
Solvents: Turpentine, a solvent distilled from pine resin, used to thin paint and speed drying.
These materials helped painters create glowing glazes, smooth brushwork and durable paint layers.
The Gamblin Approach to Oil Painting Mediums
Gamblin Artist Colours was founded in 1980 by Robert Gamblin with the goal of combining traditional oil painting knowledge with modern conservation science.
Gamblin worked with museum conservators and institutions such as the National Gallery of Art to develop mediums that maintain the working qualities artists love while improving long-term stability.
Gamblin mediums are widely respected because they:
• improve paint handling
• maintain strong paint films
• reduce unnecessary solvent exposure
• follow conservation principles used by museums
The Most Important Oil Painting Rule
Fat Over Lean
Before using oil painting mediums it is important to understand the principle of fat over lean.
Lean paint
Lean paint contains less oil and more solvent (a liquid used to thin paint).
Lean layers dry faster and are usually used at the beginning of a painting.
Example
Paint mixed with Gamsol Odourless Mineral Spirits.
Fat paint
Fat paint contains more oil or medium.
These layers dry slower and remain more flexible.
They are used in later stages of a painting.
Why this matters
Oil paint dries through oxidation (a chemical reaction where oil slowly hardens when exposed to oxygen).
If a fast drying layer sits on top of a slower drying layer, the top layer may crack as the lower layer continues to move.
So artists gradually increase oil content during the painting process.
Lean → Medium → Fat
Gamblin Oil Painting Mediums Explained
Below are some commonly used Gamblin mediums and how artists use them.
Gamsol Odourless Mineral Spirits
(a highly refined solvent used to thin oil paint)
Used for underpainting and thinning paint.
Benefits
• thins paint for sketching and blocking in shapes
• creates lean early layers
• speeds drying
• cleans brushes
Solvent Free Gel
(a medium that improves paint flow without solvents)
Benefits
• improves paint movement
• increases transparency
• reduces solvent exposure
Galkyd
(an alkyd medium — a modified oil that dries faster than traditional oil)
Benefits
• speeds drying between layers
• improves paint flow
• creates luminous glazes
- store upside down
Galkyd Lite
(a thinner version of Galkyd designed for glazing and detail work)
Benefits
• reduces brush marks
• creates smooth glazing layers
• improves paint levelling
*store upside down
Cold Wax Medium
(a mixture of beeswax and oil medium used to add texture)
Benefits
• thickens paint
• creates matte surfaces
• allows scraping and texture techniques
Gamvar Picture Varnish
(a removable protective varnish for finished oil paintings)
Benefits
• protects the painting
• deepens colour saturation
• unifies surface sheen
Gamvar can be applied once the painting is dry to the touch while the paint continues curing underneath.
Stand Oil
(linseed oil that has been heated until it becomes thicker)
Benefits
• produces smooth paint surfaces
• increases gloss
• strengthens paint films
Safflower Oil
(a pale drying oil pressed from safflower seeds that yellows less than linseed oil)
Benefits
• slows drying time
• improves blending
• keeps whites and light colours brighter
Linseed Oil vs Safflower Oil
Both linseed oil and safflower oil are drying oils (oils that harden when exposed to oxygen).
Linseed Oil
• dries faster
• creates stronger paint films
• slightly yellows over time
Best for darker colours and general painting.
Safflower Oil
• dries slower
• yellows less
• keeps colours cleaner
Best for whites and delicate colour mixtures.
Gamblin Oil Painting Medium Comparison Guide
| Medium | What it Does | Best For |
|---|---|---|
| Gamsol | Thins paint and speeds drying | Underpainting |
| Solvent Free Gel | Improves paint flow without solvents | Early colour layers |
| Galkyd | Speeds drying and adds gloss | Glazing |
| Galkyd Lite | Smooth transparent glazing | Detail work |
| Stand Oil | Thickens paint and reduces brush marks | Smooth finishes |
| Safflower Oil | Keeps colours brighter | Whites |
| Cold Wax Medium | Adds texture | Palette knife painting |
| Gamvar | Protects finished paintings | Final varnish |
A Simple Oil Painting Medium Workflow
Stage 1 – Underpainting
Paint + Gamsol
Stage 2 – Early colour layers
Paint + Solvent Free Gel
Stage 3 – Colour depth and glazing
Paint + Galkyd
Stage 4 – Fine detail and glazing
Paint + Galkyd Lite
Stage 5 – Texture effects
Paint + Cold Wax Medium
Stage 6 – Final protection
Gamvar Varnish
Frequently Asked Questions About Oil Painting Mediums
What is a medium in oil painting?
A medium is a substance added to oil paint to change how it behaves. Mediums can adjust drying time, paint flow, transparency and surface finish.
What is the fat over lean rule?
Fat over lean means each layer of paint should contain more oil than the layer underneath it. Lean layers dry faster and should be painted first.
What is Galkyd used for?
Galkyd is used to speed drying and create transparent glazes in oil painting.
What does Gamsol do?
Gamsol thins oil paint and cleans brushes while creating lean fast-drying paint layers.
Can safflower oil be used with oil paint?
Yes. Safflower oil slows drying and helps keep whites and light colours from yellowing.
Explore Gamblin Oil Painting Mediums at The Deckle Edge
At The Deckle Edge we stock a wide range of Gamblin oil painting mediums including Gamsol, Galkyd, Cold Wax Medium, safflower oil and Gamvar varnish.
These materials help artists control paint flow, drying time and surface effects while maintaining strong paint films.
If you are experimenting with oil painting mediums or trying to understand fat over lean, exploring these materials can open up many new possibilities in your painting process.
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