Straight from the tube or the pot, acrylic paint is usually fairly thick. For more fluid lines, soft transitions, a watercolour effect or fine detail, you need to thin it. But thinning it wrongly can ruin the paint's adhesion and durability. In this guide we explain how to thin acrylic paint properly with water and mediums, which to use when, and the common mistakes to avoid.
Why do we thin paint?
- For more fluid, smoother brush strokes
- For transparent, light layers like watercolour (a wash)
- For soft colour transitions and shading
- For fine detail, line work and airbrushing
- To cover large areas evenly
Water or medium? The basic difference
Both thin the paint, but the results differ:
- Water: Cheap and practical. It thins the paint and makes it flow. But too much water dilutes the binder (the glue) in the acrylic; over-thinned paint goes dull and loses adhesion and durability.
- Medium: Thins the paint while preserving the binder. In other words the paint flows, but adhesion, sheen and durability stay intact. Mediums are the choice for professional, lasting results.
The mediums among our auxiliary products are made for exactly this.
Thinning with water: the right ratio
When thinning with water, follow the rule of "add a little, check":
- 10-30% water: Makes the paint flow while largely preserving opacity. A safe range for everyday use.
- 30-50% water: Semi-transparent layers and soft transitions. Opacity drops.
- 50% water and above (a wash): A watercolour effect, very transparent. Lovely on absorbent surfaces (paper, raw wood); adhesion weakens on smooth surfaces.
On multi-layered absorbent surfaces, plenty of water isn't a problem; but on surfaces such as glass, plastic and metal, use a medium instead of water.
Thinning with a medium
Different types of medium do different jobs:
- Flow medium: Thins the paint and makes it flow, reducing brush marks.
- Glaze medium (drying retarder): Thins the paint and keeps it open; ideal for soft transitions and shading. Glaze medium is wonderful for any work that needs time to blend.
- Pouring medium: Makes the paint fluid and pourable; for acrylic pouring.
When thinning with a medium, again add it gradually and mix on the palette.
Step by step: thinning properly
1. Start with a small amount
Put a little paint on the palette. Add the water or medium drop by drop; mix and check the consistency after each addition. Adding more is easy; taking it back out is not.
2. Test the consistency
Load your brush and try it on a test surface. Adjust until you reach the flow and opacity you want.
3. Choose according to the surface
Water is fine for absorbent surfaces (paper, raw wood, canvas). For smooth surfaces (glass, plastic, metal) choose a medium; water ruins adhesion.
4. Apply in layers
Thinned paint is more transparent; reach the tone you want over several thin layers. Wait for each layer to dry.
Common mistakes when thinning
- Too much water: The paint goes dull, runs and loses adhesion.
- Adding too much at once: The consistency is lost, and it's hard to recover.
- Using water on a smooth surface: The paint won't grip and flakes off; you need a medium.
- Not mixing: Uneven consistency and a patchy result.
- The wrong medium: Each medium is for a different job; choose the right one.
Frequently asked questions
Can acrylic paint be thinned with water alone?
It can, but only up to a point. Up to 30% water is safe for most work; more than that ruins opacity and adhesion. On lasting work and smooth surfaces, a medium is healthier.
Why does paint flake off when I thin it a lot?
Too much water dilutes the binder that sticks the paint to the surface. That's why very watery paint can't hold on. The solution: use a medium.
Is a medium essential?
Not essential, but very useful for a professional, lasting and smooth result.
How do I get a watercolour effect?
Thin the paint with plenty of water (a wash) and apply it to an absorbent surface (paper, raw wood, canvas); you'll get transparent, flowing layers.
Can thinned paint be stored?
A small amount can be kept briefly in a sealed container, but using it fresh is always best.
Thinning properly multiplies what you can do with acrylic: from fluid lines to watercolour effects, from soft transitions to fine detail. Remember the rule — water on absorbent surfaces, mediums on smooth surfaces and lasting work. Take a look at our auxiliary products and unlock your paint's potential!
