How Much Catalyst to Add to Polyester Resin

Q: Practical Mixing and Safety Tips

Work in a well-ventilated area —Catalyst fumes can be unpleasant. Wear gloves and eye protection when handling both resin and catalyst. Measure ingredients accurately with digital scales to avoid mishaps. Mix thoroughly but avoid vigorous stirring to limit bubble formation. Do not return unused, catalysed resin to the original container. If you're new to resin repairs, consider an all-in-one solution like the Resin Fibreglass Repair Kit – Complete Polyester Resin and Hardener Set , which includes pre-measured components and clear instructions for safe use.

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How Much Catalyst Should I Add to Polyester Resin?

Getting the right balance of catalyst is essential for successful polyester resin repairs. Too little or too much can affect the finish, strength, and durability of your project.

Understanding Catalyst Ratios

Polyester resin requires a catalyst (commonly a hardener such as MEKP) to cure and harden. The general rule is to use between 2% and 3% catalyst by weight of resin. For example, if you are using 1kg of resin, you should add 20–30g of catalyst. Staying within this range helps ensure the resin cures properly—less may leave it tacky or uncured, more can cause it to set too quickly or become brittle.

Practical Mixing and Safety Tips

Work in a well-ventilated area—Catalyst fumes can be unpleasant.
Wear gloves and eye protection when handling both resin and catalyst.
Measure ingredients accurately with digital scales to avoid mishaps.
Mix thoroughly but avoid vigorous stirring to limit bubble formation.
Do not return unused, catalysed resin to the original container.

If you’re new to resin repairs, consider an all-in-one solution like the Resin Fibreglass Repair Kit – Complete Polyester Resin and Hardener Set, which includes pre-measured components and clear instructions for safe use.

When Precise Catalyst Ratio Matters Most

Pay particular attention to catalyst ratio when tackling structural or waterproof repairs—such as boats, pools, tanks, and rigid plastics—where improper curing may compromise the bond. Following manufacturer instructions and staying within the recommended range is crucial for reliable, durable results.

Related guides

How To Catalyse Polyester Resin

Choosing and mixing the correct amount of catalyst is fundamental for successful polyester resin projects. For more tips and a practical overview of the process, visit our main guide on how to catalyse polyester resin.