MEKP Catalyst Guide: Mixing Ratios and Safety

MEKP (Methyl Ethyl Ketone Peroxide) is the catalyst that makes polyester and vinyl ester resins cure. Understanding proper mixing ratios and safety precautions is essential for successful fiberglass work. Too little catalyst and your resin will not cure; too much and you risk dangerous exothermic reactions.

This guide covers everything you need to know about using MEKP safely and effectively.

What Is MEKP?

MEKP is an organic peroxide that initiates the chemical reaction (polymerization) in polyester and vinyl ester resins. When mixed with resin, it generates free radicals that cause the resin molecules to cross-link and harden.

Important: MEKP is NOT used with epoxy resins. Epoxy uses a completely different hardener system.

Standard Mixing Ratios

The amount of MEKP catalyst needed depends on temperature, humidity, and desired working time.

General Guidelines

• Standard ratio: 1-2% MEKP by volume
• Minimum: 0.5% (cold temperatures)
• Maximum: 3% (never exceed this)

Temperature-Based Adjustments

• Below 60F (15C): Use 2-2.5% - cold slows cure
• 60-77F (15-25C): Use 1.5-2% - ideal working range
• Above 77F (25C): Use 1-1.5% - heat accelerates cure
• Above 90F (32C): Use 0.5-1% - work in small batches

By Application

• Gelcoat: 1.5-2% for spray, 2-2.5% for brush application
• Laminating resin: 1.5-2% standard
• Tooling resin: 1-1.5% (slower cure preferred for molds)
• Casting resin: 0.5-1% for thick pours to control heat

How to Measure MEKP

Accurate measurement is critical. Use graduated mixing cups for precise ratios.

Quick Reference (per quart of resin)

• 1% = 10cc (about 2 teaspoons)
• 1.5% = 15cc (about 1 tablespoon)
• 2% = 20cc (about 4 teaspoons)

Pro Tip: Use syringes or graduated droppers for small batches. Guessing leads to problems.

Mixing Procedure

1. Pour measured resin into a clean mixing container
2. Add MEKP catalyst
3. Stir thoroughly for 1-2 minutes
4. Scrape sides and bottom of container
5. Work time starts now - apply promptly

Never add MEKP directly to a large container of resin. Always pour what you need first.

Working Time and Cure Time

Working time (pot life) and cure time are affected by:

• MEKP percentage
• Ambient temperature
• Batch size (larger = more heat = faster cure)
• Resin type and promoter level

Typical Times at 77F with 1.5% MEKP

• Gel time: 15-20 minutes
• Tack-free: 1-2 hours
• Full cure: 24-48 hours

Safety Precautions

MEKP is a powerful oxidizer and can be dangerous if mishandled.

Required Safety Equipment

• Safety glasses or goggles
• Nitrile or latex gloves
• Adequate ventilation
• Fire extinguisher nearby

Critical Safety Rules

• NEVER mix MEKP with accelerators/promoters directly - explosive reaction
• NEVER exceed 3% catalyst
• NEVER store MEKP above 80F
• NEVER return unused MEKP to the original container
• Keep away from heat, sparks, and flames
• Store in original container only

First Aid

• Skin contact: Wash immediately with soap and water
• Eye contact: Flush with water for 15+ minutes, seek medical attention
• Ingestion: Do NOT induce vomiting, seek immediate medical attention

Storage Requirements

• Store in cool, dark location (below 80F)
• Keep container tightly closed
• Store away from heat sources and direct sunlight
• Keep separate from accelerators and combustibles
• Shelf life: 6 months to 1 year when stored properly

Troubleshooting

Resin Will Not Cure

• Not enough MEKP - add more next batch
• Old or degraded MEKP - use fresh catalyst
• Temperature too cold - warm the workspace
• Resin too old - check expiration

Resin Cures Too Fast

• Too much MEKP - reduce percentage
• Temperature too hot - work in smaller batches
• Batch too large - mix smaller amounts

Soft or Sticky Cure

• Insufficient MEKP - cannot be fixed after the fact
• May need to apply additional catalyzed resin over top

Final Thoughts

Proper MEKP usage is fundamental to successful fiberglass work. Take time to measure accurately, adjust for temperature, and always prioritize safety. When in doubt, start with 1.5% and adjust based on results.

Shop MEKP catalyst and all your fiberglass supplies at Boat Suppliers.