If you’ve ever seen your acrylic paint dry on the palette before you finished blending, you know how frustrating it can be. Paint by numbers kits are meant to be a relaxing creative break, but acrylic paint dries so quickly that it can feel like you’re racing the clock. The answer is acrylic retarder medium, a simple tool that professional painters have used for years and is now becoming popular with hobbyists too.
This guide explains what acrylic retarder is, how to use it properly, and which products are worth buying in 2025.
What Is Acrylic Retarder Medium?
Acrylic retarder medium is an additive that slows down how quickly acrylic paint dries. This gives artists more time and control while painting. It is usually a water-based, colorless liquid that keeps the paint workable on your palette or canvas for longer.
Unlike water, which can make paint thinner and less vibrant, a good retarder slows drying while preserving the paint’s thickness, color, and adhesion. It does not change how the paint looks or how long it lasts once it is dry.
Numbers Artists Need a Retarder
Paint by numbers kits include small paint pots that can dry out very quickly, sometimes just minutes after you open them. This causes a few problems:
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Number Ghosting: Light colors (yellows, creams, pale pinks) don’t cover printed numbers in one coat, requiring multiple layers
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Harsh Lines: Adjacent colors leave visible boundaries instead of smooth transitions, producing a mosaic-like finish
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Wasted Paint: Paint dries in the pot before you‘ve finished a section, leading to dried-out paints and unusable colors
A retarder medium helps with all of these issues. By slowing down drying, you have more time to blend, add smooth second coats, and keep your paints usable during your painting session.
How Acrylic Retarder Works
Acrylic paint dries as water evaporates. The acrylic particles join together as the water leaves, forming a strong layer. A retarder slows down this process by adding humectants, which are compounds that attract and keep moisture.
This means you have more time to work with the paint, making it easier to use wet-on-wet techniques, blend smoothly, and create better color transitions. It also helps prevent skinning, which is the annoying film that forms on paint left on the palette.
How to Use Acrylic Retarder for Paint by Numbers (Step-by-Step Guide)
Step 1: Determine the Right Mixing Ratio
Begin with a small amount and adjust as needed. Different brands suggest different mixing ratios:
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Golden Acrylic Retarder: Approximately 7 parts paint to 1 part retarder (about 15% addition)
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General Purpose Retarders: 1 part retarder to 4–6 parts acrylic paint by volume
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Maximum Safe Ratio: Most experts recommend not exceeding 15–20% retarder to paint
A good way to start is to add one or two drops of retarder to each paint pot, mix well with a toothpick, and test it on scrap material before using it on your canvas.
Step 2: Mix Thoroughly
If you don’t mix well, the paint may dry unevenly or act unpredictably. Stir until the retarder is fully blended and the paint feels the same all the way through.
Step 3: Test Before Applying
Always try your paint and retarder mix on a spare surface or palette before using it on your artwork. This way, you can check how it feels and dries without risking your project.
Step 4: Apply in Thin, Even Layers
Retarder doesn‘t replace proper painting technique. Continue working in thin layers rather than loading on a single thick coat. The retarder simply gives you more time to execute each layer smoothly.
Step 5: Allow Adequate Drying Time
Paint mixed with retarder takes longer to dry. This is the point. However, don’t rush to varnish or frame your work immediately. Wait at least 24–48 hours, depending on humidity and the amount of retarder you used.
Recommended Mixing Ratios at a Glance
| Ratio (Paint:Retarder) | Drying Time Effect | Best For |
|---|---|---|
| 7:1 to 10:1 | Minimal extension (10–20 minutes longer) | General painting, detail work |
| 4:1 to 6:1 | Moderate extension (20–40 minutes longer) | Blending, transitions, second coats |
| 3:1 | Significant extension (40–60 minutes longer) | Wet-on-wet techniques, large areas |
Source: Consolidated from manufacturer guidelines
Common Mistakes to Avoid
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Adding Too Much: Overuse can cause the paint film to remain tacky, weaken adhesion, or even crack over time. Stick to recommended ratios.
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Using on an Unstable Surface: Make sure your paint is clean and not already dried out. Retarder does not revive dried acrylic paint—it only extends the wet stage of fresh paint.
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Skipping Tests: Different paint brands react differently to retarder. Always test before applying to your artwork.
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Ignoring Environmental Factors: High humidity reduces the effect of retarder, while very dry air can cancel out its benefits.
Practical Tips for Paint by Numbers Enthusiasts
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Pair Retarder with Other Best Practices: Always keep your paint pot lids tightly sealed when not in use. Using retarder isn‘t a substitute for proper storage habits.
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Work One Color at a Time: Open only the pot you’re actively using to minimize exposure to air.
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Moisten Your Brush: Lightly moisten the brush with retarder before picking up paint for extended working time on the canvas.
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Use a Fine Mist Spray Bottle: In especially dry environments, a light mist of water over the palette area—combined with a small amount of retarder in each pot—can further extend workability.
Best Acrylic Retarder Products for Paint by Numbers (2025)
1. Golden Retarder (237ml)
A professional-grade retarder widely recommended by artists. Recommended addition up to 15%. Works well with all acrylic paint formulations. Price: ~$12–15.
2. Liquitex Slow-Dri Blending Gel (237ml)
A gel-based retarder ideal for thicker paint and impasto techniques. Excellent for blending applications. Price: ~$10–14.
3. Mont Marte Acrylic Retarder (75ml)
Affordable and accessible for hobbyists. Suggested mix ratio is 1 part medium to 4 parts paint. Price: ~$5–8.
4. IMPRESA Acrylic Paint Thinner & Mixing Medium (8 fl oz)
Specifically noted as ideal for paint by number projects, improving coverage and blending. Price: ~$8–12.
5. Vallejo Acrylic Retarder Medium (60ml)
A precise, high-quality option for detail-oriented work. Mix up to 10% with paint. Price: ~$7–10.
Frequently Asked Questions
Can I use water instead of acrylic retarder?
Water can thin paint and briefly slow drying, but it also reduces color vibrancy and weakens adhesion. Retarder is specifically formulated to extend drying without compromising paint quality. For best results in paint by numbers, use a proper retarder.
Can a retarder revive dried acrylic paint?
No. Once acrylic paint has dried, the polymer film is permanent. Retarder only slows the drying process of fresh paint. Dried paint cannot be revived—if your paint pot has hardened, you‘ll need to purchase replacement paint.
What’s the difference between retarder and flow improver?
A retarder primarily slows drying time. A flow improver reduces surface tension for smoother application and less brush stroke visibility. While some products combine both functions, they serve different primary purposes. For paint by numbers blending needs, a retarder is usually the better choice.
How much retarder should I add to my small paint pots?
Start with one drop of retarder per paint pot (these small pots hold approximately 3–5 ml of paint). Stir thoroughly. Add a second drop if you need more working time, but remember: less is more.
Is acrylic retarder safe?
Most acrylic retarders are non-toxic and water-based. However, it‘s recommended to use them in a well-ventilated area, as some formulas can have a mild odor. Always check the specific product’s safety label before use.
Final Thoughts
Acrylic retarder medium is one of those rare tools that genuinely transforms the painting experience for paint by numbers enthusiasts. You‘ll work more relaxed, blend more smoothly, and waste less paint. Start small, follow the ratios, and test before committing—and you’ll quickly understand why so many artists consider retarder an essential part of their toolkit.
