MEKP for Polyester and Vinyl Ester Resin Systems.

If you’ve ever worked with fiberglass or composite materials, you’ve probably encountered MEKP (Methyl Ethyl Ketone Peroxide). This clear liquid catalyst is what makes polyester resin  and vinyl ester resin cure and harden — transforming it from a syrupy liquid into a solid, durable composite. Without MEKP, the resin would remain sticky and never fully set.

Understanding how MEKP works, how to use it safely, and how to avoid common curing issues can mean the difference between a professional-grade finish and a failed project. Whether you’re repairing a boat transom, laminating a mold, or building a custom part, mastering catalyst ratios and conditions is key.

This guide explainshow MEKP works, how to handle it safely, and how to optimize your mix for the best results.

What Is MEKP and How Does It Work?

MEKP is an organic peroxide that starts the polymerization process in polyester and vinyl ester resin. When added, it breaks down and releases free radicals that trigger cross-linking between polymer chains — converting the liquid resin into a solid thermoset plastic.

Once cured, polyester and vinyl ester resin cannot be melted or reshaped, which is why accurate measuring and timing are critical.

Because this process is exothermic (heat-producing), the amount of heat depends on:

·      The volume of resin being catalyzed

·      Theamount of MEKP used

·      Theambient temperature

In large pours or thick laminations, this heat can build quickly — sometimes causing cracks, discoloration, or even smoking resin. Managing exotherm is one of the most important parts of a successful cure.

Safety and Handling Tips

MEKP is a strong oxidizer and should always be handled with caution.

Essential safety practices:

·   Wearchemical-resistant gloves,safety glasses, and arespirator or mask when needed.

·      Work in a well-ventilated area or use exhaust fans.

·      Store MEKP in a cool, dry place away from sunlight and heat.

·      Keep it tightly sealed in its original container.

·      Never store near flammable materials or accelerators like cobalt naphthenate.

Never mix MEKP directly with an accelerator. This can cause an explosive reaction. Most resins already contain the necessary promoter—your job is simply to add the catalyst and mix thoroughly.

If you spill MEKP, neutralize it with baking soda (sodium bicarbonate) before wiping. Dispose of all contaminated materials according to local hazardous waste regulations.

Catalyst Ratios and Temperature Guidelines

The correct MEKP, ratio typically ranges from 1% to 2% by volume of resin. Adjust based on temperature and project size:

Mix gently but completely for at least one full minute, scraping sides and bottom of the container. Incomplete mixing causes soft spots and uneven curing.

If humidity is high, surface blush (a waxy film) may form. To prevent it, use wax-added resin or apply PVA (polyvinyl alcohol) on the surface.

Common MEKP Issues & How to Fix Them

Getting the Best Results

MEKP is the engine behind polyester and vinyl ester resin systems — it’s what turns liquid resin into a tough, reliable composite.

For success every time:

·      Measure accurately using dedicated tools.

·      Mix completely but gently to avoid bubbles.

·      Adjust for temperature — never guess your ratios.

·      Work safely and store MEKP properly.

Whether you’re building a custom part, restoring a boat, or performing a transom repair, understanding catalyst behavior helps you produce consistent, professional-quality results.

Explore our fiberglass repair kits and resin systems at True Composites to find everything you need for your next project.