Short answer: A medium is a transparent additive you mix into paint without adding any pigment (the colour particles). It changes how the paint flows, how transparent it is, and what finish it leaves; but unlike water, it keeps the binder intact (the glue that sticks paint to the surface). That is why a medium is the shortest route to clean, ring-free coats when glazing (transparent colour transitions) or washing (the shading fluid that pools in recesses).
Here is the difference in one line. Water dilutes the pigment, but it dilutes the binder too; the paint behaves "like water," runs into recesses, and leaves a coffee ring (a tide mark, or drying line) at the edge. A medium makes the pigment transparent while keeping it sticky. That is exactly why most painters treat a medium as an invisible but critical tool.
What is a medium and what is it for?
Let's start where the paint itself starts. Acrylic paint is made of three things: pigment, binder, and water or medium. A medium is the clear fluid you add to that mix without adding colour. It thins the paint without "weakening" it, because it carries its own binder.
So what does that buy you in practice? A medium is good for:
- Making paint transparent: It dilutes the pigment but keeps the glue, so you get a clean, blotch-free transparent coat. This is the foundation of Glazing.
- Flow control: It sharply reduces tide marks and uneven drying.
- Adjusting finish: Matt mediums (like Lahmian) give a matt finish; others give satin or gloss.
- Drying time: A retarder extends drying time, so you can keep working the paint on the surface for longer.
So in short, a medium is how you swap water for a smarter version of itself. It adds no colour, only behaviour.
Why use a medium instead of water?
This is the most misunderstood part of mediums, so it's worth unpacking. Water is free and always on hand, so most people try to glaze and wash with water alone. The catch: as the water goes up, the binder ratio in the mix goes down.
And when the binder drops, three things happen. The paint runs uncontrolled into recesses; it leaves a coffee ring at the edge; and it grips the surface weakly, so it lifts easily when you apply the next coat. A medium carries its own binder, so it keeps the pigment adhering and evenly spread as you thin it.
The core principle to remember: a medium doesn't lighten colour, it makes it transparent. If you want to lighten a tone, you add white; if you want to make it see-through, you add medium. They are two completely different jobs.
That distinction matters across every transparent-colour task, including Washing, Shading & Pin Wash and glazing. Put simply, water punishes you with rings and blotches; a medium rewards you with a clean coat.
Which medium does what?
The market is full of fluids sold as "medium" that do very different jobs, so it helps to keep them straight. Here are the most common ones and what each is for:
- Lahmian Medium (matt): The classic glazing tool. It makes paint transparent for glazing and washing, thins it without crushing colour saturation, and reduces tide marks.
- Contrast Medium: For thinning Contrast paints to a lighter tone or glaze. It preserves the "one-coat" flow (the way Contrast covers in a single pass) and lets you mix your own custom Contrast colour.
- Glaze Medium (generic): A brand's own glaze medium (Vallejo, for example). Same job, that is, making paint transparent.
- Flow improver: Lowers surface tension and improves how the paint flows and spreads. It makes a real difference for airbrushing and fine lines.
- Retarder: Extends drying time. It's almost essential for Wet Blending.
- Matt / gloss medium: Adjusts finish. Gloss for gems and lenses, matt for large surfaces.
Step by step: how to use a medium for glazing
The logic of glazing with a medium is simple: put the medium down first, then add a tiny bit of paint. The pigment ratio must always stay low. Here's the order:
- Put medium on the palette. Start by putting some medium on the palette. You don't start with water; the body of the mix will be medium.
- Add a tiny bit of paint. Stir in the barest amount of paint. The goal is a low-pigment mix; colour should build over several coats, not appear in one.
- Mix and check consistency. Aim for a transparent, fluid but "sticky" glaze. Unlike a water mix, this one won't drip and scatter.
- Apply a thin coat. Touch the brush to a paper towel to shed the excess, then lay on a thin coat. The tide-mark risk is far lower than with water.
- Layer it. Apply the next coat only after the last one has fully dried. Depth builds gradually; rush it and you'll lift the coat underneath.
For a very thin glaze you don't have to ban water entirely; a little water plus medium works too. Just let the medium always carry the body of the mix.
Where do I use mediums?
A medium isn't the tool of one technique; it's the quiet partner of several workflows. Here's where you'll meet it most:
- Glazing and washing: All transparent-colour work. This is the medium's home.
- Wet blending: A retarder extends how long the paint stays wet on the surface, which gives you time to blend two colours in place.
- Two thin coats: Thinning with medium is an alternative to water; for details, see Thinning Paint: Two Thin Coats.
- Contrast highlights: With Contrast Medium you can turn a Contrast paint into a "highlight glaze" and build lighter transitions.
- Airbrushing: Flow improver and retarder help heavily thinned paint flow through the needle without clogging.
Most common mistakes
Most medium mistakes come from expecting the wrong thing from it. The usual suspects:
- Glazing and washing with water only. Tide marks and uneven drying are then inevitable; a medium solves this directly.
- Thinking a medium lightens colour. A medium doesn't lighten, it makes transparent. Add white to lighten, medium to make see-through.
- Adding too much retarder. The paint never dries, turns slimy, and even takes a fingerprint. So use retarder sparingly.
- Choosing the wrong finish medium. Matt on a gem or gloss on a large surface both ruin the look. Always match the finish to the job.
- Mixing in too much pigment. Even with medium, if you don't keep the paint low the glaze goes opaque; that's no longer a glaze, it's a bad layer.
Pro tips
Getting good results from a medium hides in a few small habits:
- Make medium a habit for glazing and washing. That single change alone visibly cleans up your transparent-colour work.
- Use Contrast Medium to turn Contrast paints into a "highlight glaze" for lighter transitions; a glaze that lifts rather than darkens.
- Add retarder drop by drop, because a little goes a long way. Try the smallest amount first and increase only if needed.
- For a very thin glaze, try water plus medium; pure medium sometimes has too much "tooth" (drag you feel as you brush), and a little water smooths the flow.
- Mixing medium into a wash makes even a ready-made wash more controlled and less ring-prone; thin an over-saturated wash with medium.
Frequently Asked Questions
What's the difference between a glazing medium and water?
The key difference is the binder. Water dilutes the pigment but dilutes the binder too; the paint behaves like water, runs into recesses, and leaves a coffee ring. A medium contains binder, so it keeps the pigment adhering and evenly spread as it thins. The result: a glaze made with medium comes out far cleaner, blotch-free, and ring-free.
What exactly does Lahmian Medium do?
Lahmian Medium is a matt glazing medium. It makes paint transparent for glazing or washing, thins it without crushing colour saturation, and reduces the risk of tide marks. It's the classic glazing tool, and it largely eliminates the blotching and rings you get when thinning with water alone.
Can I use Contrast Medium like a normal medium?
Not exactly. Contrast Medium is designed to preserve the one-coat flow of Contrast paints. With it you thin a Contrast paint to a lighter tone or glaze, or even mix your own custom Contrast colour. For ordinary glazing, though, Lahmian or a generic glaze medium is a better fit.
Does a medium lighten the paint colour?
No, and this is the most common misconception. A medium doesn't lighten colour, it makes it transparent. If you want to lighten a tone, meaning make it paler, you add white or a lighter colour. Adding medium makes the colour see-through, not lighter. Confuse the two and you won't get the result you want.
How much retarder should I add?
Very little. Retarder extends drying time and is great for wet blending, but too much stops the paint from drying at all; the surface stays slimy and even takes fingerprints. So add it drop by drop, try the smallest amount first, and increase only if needed. A little retarder is usually more than enough.
Do I need a medium for washing too?
Not necessarily, because ready-made washes already contain medium and flow agents. Still, thinning an over-saturated wash with medium makes it more controlled and less ring-prone. And if you mix your own wash from paint, using medium gives a far cleaner result than water alone.
How do I choose a finish medium?
By the job. Use a gloss medium on surfaces you want shiny, like gems, glass, and lenses. On large skin, cloth, and general surfaces, a matt medium looks more natural. The wrong finish, such as gloss on a large surface, creates unwanted reflections and a plastic look.
Key takeaway: A medium is water that got smart; it thins the pigment but keeps the binder, so your glazes and washes come out ring-free and clean. Match the right medium to the right job (Lahmian for glazing, retarder for wet blending, gloss for gems), and above all, never confuse lightening colour with making it transparent.
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