Polyester laminating resin needs the correct amount of methyl ethyl ketone peroxide (MEKP) catalyst to cure. Too little can leave a soft or under-cured laminate. Too much can shorten the working time, increase heat and create a dangerous reaction. The correct percentage is the one stated for your exact resin at the actual shop temperature.
This guide explains the workflow around the ratio rather than inventing a universal recipe. Resin formulations, catalyst strengths, ambient temperatures and laminate masses differ. Treat the current product data sheet as the controlling instruction and use a scale or graduated dispenser that can measure the required amount accurately.
Quick answer
Do not estimate MEKP by eye and do not assume one fixed percentage works in every season. Confirm the manufacturer’s permitted range, calculate the batch, measure accurately and make a small test before committing a large laminate.
What you need to know
- MEKP is a reactive organic peroxide, not simply a drying additive.
- Shop temperature, mould temperature and batch depth affect working and cure time.
- Catalyst is measured against resin quantity, but the manufacturer may specify weight or volume. Do not interchange them casually.
- Large masses retain heat. Several controlled batches are safer and easier to laminate than one oversized pot.
Tools and materials
- Exact polyester resin and compatible MEKP specified by the resin supplier
- Manufacturer’s current technical and safety data sheets
- Accurate scale or compatible graduated catalyst dispenser
- Clean mixing cups and flat-sided mixing sticks
- Fibreglass reinforcement cut and dry-fitted before mixing
- Rollers, brushes or squeegees suited to the laminate
- Eye protection, compatible gloves, protective clothing and ventilation controls
Step-by-step method
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Prepare the job before catalysing
Cut reinforcement, mask adjacent surfaces, set out tools and plan the application sequence. Once MEKP is mixed into resin, the usable clock has started. Remove ignition sources and keep the work area clean and ventilated.
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Record actual conditions
Measure ambient, material and mould temperature. Compare those conditions with the product instructions. If the shop is outside the permitted range, condition the materials or postpone the work rather than correcting by guesswork.
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Calculate a manageable batch
Choose a resin quantity you can wet out within the stated working time. Calculate the catalyst from the exact percentage and measurement method in the data sheet. Write the calculation down and label the cup if more than one batch will be mixed.
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Measure resin, then catalyst
Dispense the resin first. Measure MEKP with dedicated equipment and prevent contamination of the catalyst container. Never return dispensed catalyst to its original package, and never allow MEKP to contact accelerators or promoters directly.
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Mix thoroughly without whipping air
Scrape the cup walls, bottom and the mixing stick while blending. A clear resin may not show streaks, so use a consistent timed mixing method. Do not leave the mixed resin sitting in a deep pot longer than necessary.
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Wet out and consolidate
Apply the resin to the prepared surface, lay the reinforcement and work resin through the fabric. Use a consolidating roller or squeegee to remove trapped air without starving the laminate. Add only enough resin to wet the reinforcement fully.
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Monitor cure and document the batch
Record resin lot, catalyst lot, ratio, temperature, start time and observations. Allow the laminate to cure undisturbed. If it remains tacky or soft beyond the published cure window, isolate it and consult the manufacturer before adding another layer.
Common mistakes
- Adding extra catalyst to compensate for cold conditions
- Mixing a large bucket that gels before it can be used
- Confusing MEKP with MEK solvent
- Using contaminated measuring equipment
- Applying resin over wax, moisture, dust or an incompatible cured surface
- Assuming a tacky laminating-resin surface is always a failed cure without checking whether the product is air-inhibited
Safety
MEKP can seriously injure eyes and skin and can decompose violently when contaminated or heated. Store and handle it exactly as the supplier directs, away from heat, sunlight, incompatible materials and ignition sources.
Have the required eyewash and spill response available. Do not use food containers or household measuring tools. For workplace use in Canada, follow WHMIS requirements and the product SDS.
Frequently asked questions
How much MEKP should I add?
Use only the range and method published for your exact resin and catalyst. A generic percentage copied from another product is not reliable enough for safe production work.
Can I add more catalyst to make resin stronger?
No. Catalyst controls the cure reaction; it is not a strength additive. Moving outside the permitted range can reduce performance and increase heat and safety risk.
Why did my resin cure too quickly?
Common causes include a warm shop, warm materials, excessive catalyst, an oversized batch or a deep mixing container that retained heat.
Why is the laminate still soft?
Possible causes include insufficient or poorly mixed catalyst, low temperature, contamination or an incompatible resin/additive. Do not simply coat over it; identify the cause first.
Get the right materials for the job
Tell our team what you are making and how the finished part will be used.
Shop polyester resinsAsk our technical teamView the original printable project sheet.
Sources and further reading
This guide provides general educational information. Products, substrates and working conditions vary. Follow the current technical data sheet, package directions and safety data sheet for every material you use. Structural, code-regulated or safety-critical work may require a qualified professional.
