How to Thin and Mix Acrylic Paint for Paint by Numbers
The paints in a paint-by-numbers set are usually of the right consistency — most of the time. But if you open the jar and the contents resemble paste more than paint, or if you notice the color running out during painting, there's no need to panic. Thinning and mixing acrylic paint are simple skills that every paint-by-numbers artist learns quickly.
In this guide, we will cover the essential aspects for paint-by-numbers: how to thin the paints that come with your set, how to mix new shades, and how to save thickened paint before it's too late.
When should acrylic paint be thinned?
In paint-by-numbers sets, the paints are generally ready to use. Thinning is typically needed in three situations:
The paint has thickened in the jar. If the jar has been even slightly open, air dries the paint on the surface. The paint may still be usable — it just needs a little help.
You want a thinner layer. In some areas, a thinner layer of paint gives a better result — for example, when painting a light shade over a dark one and you want to build coverage with multiple layers.
The paint does not spread evenly. If the brush leaves streaks and the paint surface looks uneven, the paint may be too thick for the current conditions. Hot room air and low humidity dry acrylic paint faster.
Thinning acrylic paint with water — the right way
The simplest and safest way to thin acrylic paint from a paint-by-numbers set is with plain water. Acrylic paint is water-based, so water works naturally.
Here's how to do it:
- Dip a toothpick or the end of a brush in water. Do not pour water directly into the jar — you'll get too much at once.
- Add 1–2 drops of water to the paint jar. Always start with a small amount.
- Mix with a toothpick down to the bottom. Stir thoroughly so that no thick paint remains at the bottom of the jar.
- Test with a brush. The paint should spread evenly without streaks but remain thick enough to cover the canvas.
- Add more water if needed. One drop at a time, no more.
How much water is too much?
Rule of thumb: water should be no more than 20–30% of the paint's volume. In practice, this means a few drops in the small jars used in paint-by-numbers, no more.
Excessive water breaks down the structure of the acrylic binder. The result is a translucent, weakly adhering layer of paint that does not cover the numbers and does not stand the test of time. If the paint starts to look milky or runs off the brush like water — you've added too much.
Alternative to water: acrylic mediums
If you want to thin the paint without losing coverage, an acrylic medium (flow aid or thinning medium) is a better choice than just water. The medium thins the paint's consistency but retains the binder, so the color remains bright and opaque.
Flow aid (flow medium) is especially handy for paint-by-numbers enthusiasts. You only need a drop or two per jar, and it makes the paint smoother without affecting the pigment.
You don't need to buy the medium separately for each project — one small bottle is enough for dozens of paintings. Affordable options can be found in art stores.
Mixing colors in a paint-by-numbers set
In a paint-by-numbers set, each area has its own number and color. But what if you run out of paint, or want to fine-tune a shade?
When you run out of paint
This is the most common reason for mixing colors in a paint-by-numbers set. Small areas might remain unpainted when a jar runs out prematurely.
Solution 1: Mix from nearby shades. Check the painting's legend to see which numbers are close to each other on the shade chart. Often, adjacent numbers belong to the same color family — combining them will get you close to the original.
Solution 2: Mix from primary colors. If your set has red, blue, and yellow (or their shades), you can mix almost any color:
- Red + yellow = orange
- Blue + yellow = green
- Red + blue = purple
- Add white for a lighter shade, black for a darker one
Solution 3: Separate acrylic paint set. If many shades are missing, a 24-color loose paint set gives more flexibility. However, note that the shades may differ slightly from the originals.
Basic rules for mixing
- Always mix on a separate surface — not directly in the jar, to avoid ruining the original color. A paint jar lid, plastic lid, or piece of aluminum foil works.
- Start with the lighter color and add the darker one gradually. Lightening a dark color requires much more paint than darkening a light one.
- Mix enough at once. Hitting the same shade again is difficult, so it's better to make too much than too little.
- Test on paper before applying to canvas. Wet acrylic paint looks different from dry — check the dried result.
Saving thickened paint
Has the paint thickened but not yet completely dried? Good — it can still be saved.
Slightly thickened paint (sticky but moves)
- Add 1–2 drops of water
- Close the jar and let it sit for 5–10 minutes
- Stir with a toothpick — the paint often evens out on its own
Clearly thickened paint (stiff but not hard)
- Add 2–3 drops of water
- Stir and crush the thick lumps with a stick
- Close the jar and let it sit for 15–30 minutes
- Stir again. Repeat if necessary.
Almost dry paint (hard but not rock-hard)
- Add a few drops of water and let it soak overnight with the jar closed
- The next day, stir and check the consistency
- If it doesn't soften — the paint's binder has broken down and it's best to replace it
Tip: If the paint dried completely and doesn't revive with water, see the dry paint rescue guide, where we go through alternative options.
Acrylic paint drying time — take advantage of it between layers
The drying time of acrylic paint is one of its best features for paint-by-numbers artists. A thin layer is touch dry in about 10–20 minutes, a thicker layer in 30–60 minutes. Complete drying (the entire paint layer through and through) takes a few hours.
Why are layers worth it?
In paint-by-numbers, the numbers and lines often show through the first layer of paint — especially with light shades. The solution is not thicker paint, but multiple thin layers:
- Paint the first thin layer
- Let it dry for 15–20 minutes (paint other areas in the meantime)
- Paint the second layer — the numbers disappear and the surface is even
This technique works particularly well with white, yellow, and other light shades, which naturally have weaker coverage. Learn more about light paint and improving coverage.
Factors affecting drying
- Room temperature: A warm space speeds up drying. 20–25 °C is ideal.
- Humidity: In dry air, paint dries faster — good between layers, bad for an open jar.
- Layer thickness: A thin layer dries in minutes, a thick one can take an hour.
Finishing the completed painting with varnish
When Paint By Numbers is complete and properly dried (let it dry for at least 24 hours), finishing varnish will protect your work and give it a professional look.
Varnish protects the paint from dust, UV light, and scratches. Glossy varnish enhances color depth, while matte gives a subdued finish.
When applying varnish, the same rules as painting apply: one thin, even layer at a time. Allow drying between layers.
Common mistakes and how to avoid them
| Mistake | Why it's a problem | Solution |
|---|---|---|
| Too much water at once | The paint loses coverage and binder | One drop at a time, test |
| Mixing directly in the jar | Ruins the entire color if the shade is wrong | Always mix on a separate surface |
| Throwing away thickened paint | Paint is often salvageable | Try water + overnight soaking |
| Second layer on top of wet | Colors mix and the surface wrinkles | Wait 15–20 min between layers |
| Leaving the jar open while painting | Drying starts immediately | Close the jar whenever you're not actively using it |
Tools that make it easier
Paint-by-numbers sets it usually comes with everything you need: canvas, paints, and brushes. These accessories make painting smoother:
- Brushes — Finer brushes for small areas, wider ones for flat surfaces
- Correction pen — Covers translucent numbers in light areas
- Finishing varnish — Protects the finished work and enhances colors
- Toothpicks or cocktail sticks — For mixing and adding water
Frequently Asked Questions
Can the paint in a paint-by-numbers set be thinned with water?
Answer: Yes. The paint-by-numbers sets use water-based acrylic paint that can be thinned with regular water. Add water one drop at a time and mix well. Avoid adding more than 20–30% water to the paint volume, otherwise the paint loses its opacity.
How quickly does acrylic paint dry in a paint-by-numbers set?
Answer: A thin layer is touch-dry in about 10–20 minutes, a thicker layer in 30–60 minutes. Complete drying takes a few hours. Paint other areas in the meantime — this way you use the drying time efficiently.
How can I make light paint more opaque?
Answer: Paint multiple thin layers instead of one thick layer. Allow each layer to dry for 15–20 minutes before the next. Two thin layers cover numbers better than one thick one, and the result is more even. More tips on light paint.
Can different colors be mixed in a paint-by-numbers set?
Answer: Yes, acrylic colors mix well with each other. Always mix on a separate surface (lid or foil), not directly in the jar. Start with the lighter color and gradually add the darker one. Test the dried result on paper before painting on the canvas.
Does the finished Paint By Numbers need varnishing?
Answer: Varnishing is not mandatory, but it protects the painting from dust, light, and scratches — especially if you hang the work on the wall. Finishing varnish also enhances the depth of colors and gives the painting a finished look.
