How Much Thinner Should You Add to Polyester Resin?

Q: Practical Dilution and Mixing Tips

Gradual addition: Add thinner slowly, mixing thoroughly between additions, to monitor consistency and avoid overshooting the recommended percentage. Use the right thinner: Always choose a compatible polyester resin thinner, rather than generic solvents, to avoid unpredictable results. Prepare for curing: Excessive thinning can cause slower curing times, sticky surfaces, or weakened bonds. Never exceed 10% thinner unless the manufacturer’s instructions state otherwise. If working on repairs where strength is essential—such as using a ready-to-use Resin Fibreglass Repair Kit —dilution is typically unnecessary and not recommended, as these products are formulated for direct use.

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How Much Thinner Should I Add to Polyester Resin?

Finding the right balance when adding thinner to polyester resin is crucial for success. Using too little may leave your resin hard to apply, while too much can affect performance. Here’s what you need to know about diluting polyester resin safely.

Recommended Thinner Ratios

As a general guideline, you should add between 2% to 10% thinner by volume to polyester resin. Staying within this range helps maintain the resin’s desired workability without seriously weakening the cured product or impeding its ability to set. Always refer to the manufacturer’s technical sheet for your specific resin, as requirements may vary.

Practical Dilution and Mixing Tips

Gradual addition: Add thinner slowly, mixing thoroughly between additions, to monitor consistency and avoid overshooting the recommended percentage.
Use the right thinner: Always choose a compatible polyester resin thinner, rather than generic solvents, to avoid unpredictable results.
Prepare for curing: Excessive thinning can cause slower curing times, sticky surfaces, or weakened bonds. Never exceed 10% thinner unless the manufacturer’s instructions state otherwise.

If working on repairs where strength is essential—such as using a ready-to-use Resin Fibreglass Repair Kit—dilution is typically unnecessary and not recommended, as these products are formulated for direct use.

When This Matters

Adjusting the amount of thinner matters most when you need to increase the resin’s flow for detailed moulds or thin film applications. It is less relevant for simple repairs, where undiluted kits ensure maximum strength and convenience.

Related guides

How Do You Dilute Polyester Resin

For most home and DIY uses, choosing a polyester resin kit designed to be ready-mixed is often the safest route. To learn more about effective dilution and safe handling, see our main guide at How Do You Dilute Polyester Resin?