Resin Additive vs Thinner: Key Differences for Polyester Resin

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What’s the Difference Between a Resin Additive and a Thinner?

If you’re working with polyester resin, you might have come across the terms “resin additive” and “thinner.” Though both can be used during resin projects, they serve very different purposes. Knowing how each one works ensures you achieve the desired result, whether you’re creating art or tackling structural repairs.

What Is a Resin Additive?

A resin additive is designed to alter the appearance or certain minor behaviours of resin without affecting its fundamental properties like curing or viscosity. For example, Resi-Blast is a popular additive in the art world. It creates unique cell effects and visual textures when dropped onto the liquid resin, perfect for resin art and creative projects. Additives generally don’t make the resin easier to spread or change how long it takes to set, making them ideal for enhancing looks rather than performance.

What Does a Thinner Do?

Thinners, on the other hand, are used to reduce the viscosity of polyester resin. This means the resin becomes more fluid and easier to apply, especially when you need it to penetrate surfaces or soak into materials like fibreglass mat. However, using a thinner can impact the final strength, cure time, or finish of your project, so it’s important to use thinners with care and only if the resin manufacturer recommends it for your intended purpose.

When This Matters: Choosing the Right Approach

If your goal is to achieve decorative effects—like the striking cell patterns seen in resin art—then opt for an additive such as Resi-Blast. For functional repairs or laminating (e.g., with a fibreglass repair kit), it’s generally best to use resin as supplied, without dilution, unless specified otherwise by the product instructions. Each solution serves distinct needs, so choose with your project’s requirements in mind.

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How Do You Dilute Polyester Resin

Understanding the roles of resin additives and thinners helps you select the right products for every resin project. For more guidance on using and optimising polyester resin, explore our full dilution and application FAQ.