Everything You Need to Know About Oil Painting Mediums
What is a Medium Exactly?
A medium is any additive that modifies the consistency or drying speed of oil paint. The paint straight from the tube is pigment + oil binder. You can alter it in two main directions:
- Add solvent (like OMS or turpentine) → thins the paint, makes it matte, and speeds drying.
- Add oil (like linseed, walnut, or stand oil) → makes the paint smoother, glossier, and slower to dry.
We create a basic medium out of a mix of these two main additives : SOLVENT + OIL
Too much solvent is not good for the consistency of the paint layer, we don’t use it alone past the underpainting and too much oil can create drying issues so we try to avoid excessive fat in our paint film (remember that there’s already some plenty of oil in your paint tube), so we use the best of the both worlds and generally make mediums out of a solvent and an oil.
👉 Simple Recipe: ⅓ Linseed Oil + ⅔ Odorless Mineral Spririts (Solvent)
From there we can incorporate some more additives to make the medium behave differently, like resins to bring more gloss or siccatives to make it dry faster. This is where it gets very complicated but that’s not really the point for us today.
The best is always in the simplest things. Much more important is to understand how to properly use mediums to layer the painting.
Layering Properly with Mediums
Both “fat over lean” and “thick over thin” describe how to build layers that dry correctly in oil painting — and your choice of medium plays a huge role.
1. “Fat Over Lean” — Controlling Oil Content
“Fat” means more oil; “lean” means less oil (or more solvent).
Oil paint dries through oxidation, not evaporation — the more oil, the slower it dries.
If you put a lean, fast-drying layer over a fat, slow-drying one, the top layer will dry first, shrink, and eventually crack as the layer beneath continues to move.
✅ Rule of thumb: Each new layer should contain slightly more oil (or medium) than the one before.
Mediums help you do this gradually — start with a lean mix (more solvent), and as you move upward in the painting, add more oil or use a fatter medium mix.
My favorite trick is to use several bottles prepared with increasing oil ratio : I start with bottle number one and finish with number 5.
2. “Thick Over Thin” — Controlling Paint Texture
The “Thick of Thin” rule refers to the physical build-up of paint rather than its oil content.
Thick paint naturally dries slower than thin paint because air can’t reach the inner layers as easily.
✅ Rule of thumb: Apply thicker impasto layers on top of thinner, more diluted underpaintings.
If you reverse this (thick paint below, thin glaze above), the surface may wrinkle or crack as the base continues to dry and shift underneath.
3. How Mediums Help
Mediums are your control tools for both of these processes.
- Add solvent to make paint leaner and thinner for early layers.
- Add oil or add a gel medium to make paint fatter and thicker for upper layers.
Which Medium Should You Actually Use?
If you’re starting out, keep it simple.
Too many mediums lead to unpredictable drying, yellowing, or cracking. You don’t need a chemistry set — just a basic setup that fits your pace.
Here are two solid options for beginners:
Option 1 – The Traditional Medium:
- Use a little OMS or turpentine for the first thin layer (“lean”).
- Then use a simple medium made of 1/3 linseed oil and 2/3 OMS after the underpainting.
- For each new layer, increase the amount of oil to your medium.
Option 2 – The Modern Option:
- Use a small amount of alkyd medium (like Galkyd or Liquin) throughout the painting.
- For each new layer, increase the the amount of alkyd slightly (it makes the paint film more flexible)
- This will make your paint film dry faster.
Both approaches work — choose based on your temperament: patient and traditional, or fast and efficient.
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