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Getting the right amount of hardener is crucial for a strong, reliable polyester resin bond. Read on for practical ratios, mixing tips, and why careful measurement matters.
Ideal Hardener Ratio
For most polyester resin applications, add between 2% and 3% hardener (usually a methyl ethyl ketone peroxide, or MEKP) by weight of resin. As a practical guide, if you are working with 100g of resin, you should add 2g to 3g of hardener. Exceeding this range can cause issues such as incomplete curing or weakening of your finished part. Always refer to the specific instructions provided with your resin or chosen repair kit for the most reliable results.
Mixing Tips for Best Results
Measure both the resin and hardener as accurately as possible. Use digital scales for best accuracy, especially for small batches. Add the hardener to the resin and mix thoroughly, taking care to blend right to the sides and bottom of your container. Thorough mixing ensures that the hardener is evenly distributed, leading to a uniform cure. If you’re using a ready-to-use set such as the Resin Fibreglass Repair Kit, you’ll find pre-measured components with instructions, making this process straightforward.
When Precise Measurement Matters
Adding too little hardener may result in a sticky, soft surface that doesn’t fully cure, while too much can lead to rapid setting before you’ve finished working. Accurate measurement matters most for structural repairs such as boats, swimming pools, and rigid plastics, where the quality of the cured resin directly affects the durability and water resistance of your repair.
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Measuring hardener accurately makes all the difference to the performance of your polyester resin. For more advice on catalysing polyester resin and achieving professional results, see our main guide: How do you catalyse polyester resin?