MEKP Catalyst for GRP Roofs: Dosing, Safety, and Why It's the Most Dangerous Bottle on Site
UK guide to MEKP (methyl ethyl ketone peroxide) catalyst for fibreglass roofing: temperature dosing tables, mixing rules, storage, and why bucket exotherm starts roof fires.
A 30-foot fireball ripped through a UK fibreglass factory in 1979 because an employee dropped a rag soaked in MEKP. The catalyst that hardens your flat roof, the bottle every Cure It and Topseal kit ships with, is one of the most reactive chemicals a homeowner will ever handle on a building site. Get the dose wrong and the roof either never cures (and stays sticky for months) or smokes in the bucket before you can spread it. Store it next to the wrong chemical and you can blow up your shed.
This page is the safety and dosing reference that the rest of your GRP fibreglass roof depends on. If you're hiring a roofer, you don't need to mix anything yourself, but you do need to know what they should be doing and what failure looks like so you can spot it before the topcoat goes on and the failure gets sealed in.
What MEKP actually is
MEKP stands for methyl ethyl ketone peroxide. It's a viscous, slightly oily liquid, usually clear to faintly yellow, sold in plastic bottles with childproof vented caps. In the UK it's most commonly the Butanox M50 formulation made by Nouryon (a Dutch chemicals firm) or rebadged generic equivalents from Cure It, Stronghold, CFS Fibreglass, and others. The "50" refers to the active peroxide concentration, around 50% in a phthalate-based plasticiser carrier.
It's a catalyst, which in chemistry means it triggers a reaction without being consumed by it. When MEKP is mixed into polyester resin, it generates free radicals that start the cross-linking reaction that turns the liquid resin into a hard, waterproof solid. Without MEKP, polyester resin would sit in the bucket forever. With too much, it cures so fast it cooks itself in the bucket and can catch fire. With too little, it never quite finishes curing and you end up with a tacky roof that grabs anything that touches it.
Don't confuse MEKP with MEK. MEK (methyl ethyl ketone) is a common solvent used for cleaning brushes and welding PVC. MEKP (methyl ethyl ketone peroxide) is an organic peroxide that can ignite spontaneously when contaminated. They sound similar. They're not interchangeable, and treating MEKP with the casual respect you'd give a tin of MEK is how people get hurt.
Under UK retained EU CLP Regulations, MEKP carries four serious hazard classifications:
- H225, highly flammable liquid and vapour
- H242, heating may cause a fire (this is the bucket exotherm hazard)
- H302, harmful if swallowed
- H314, causes severe skin burns and eye damage
It's also classified as an oxidising agent, which means it can intensify any fire it's near and can react violently with reducing agents and accelerators. COSHH applies. So does the DSEAR (Dangerous Substances and Explosive Atmospheres Regulations) framework if you're storing more than a few bottles. For domestic DIY quantities (a 250ml or 500ml bottle), the risk assessment is simpler but the chemistry is identical.
Why the dose matters
MEKP isn't a "more is better" situation. Adding extra catalyst doesn't make a roof cure faster past a certain point. It makes the resin generate more heat as it cures, which accelerates the cure further, which generates more heat. That's a runaway reaction called an exotherm. In a thin laminate spread on a roof deck, the heat dissipates and the cure stays controlled. In a half-full bucket of catalysed resin that the roofer didn't get out fast enough, the heat has nowhere to go. The mixture starts to smoke. Then it catches fire. This isn't theoretical. Multiple BuildHub threads document buckets going up on real jobs.
Under-dosing is the opposite failure mode. Below 1% catalyst, the cross-linking reaction never gets enough free radicals to complete. The resin stays partly liquid. The laminate looks cured on top but flexes underneath and never bonds properly to the OSB3. Six months later the topcoat starts lifting, water gets underneath, and the whole roof has to come off.
The right dose is determined by ambient temperature. Hotter weather means the cure runs faster on its own, so you need less catalyst. Colder weather needs more. Direct sunlight on a dark deck adds 10-15°C above ambient air temperature, which is why experienced roofers reduce catalyst slightly on bright summer days even when the air feels cool.
The Cure It branded dosing table (4 bands)
Cure It is the dominant branded GRP system in UK builders' merchants. Their dosing table for Cure It catalyst paired with Cure It resin uses four temperature bands:
| Temperature | Cure It catalyst dose | Working time |
|---|---|---|
| 0-11°C (Winter grade) | 4% by weight | 20-30 min |
| 12-17°C (Winter or Summer grade) | 3% | 25-40 min |
| 18-22°C (Summer grade) | 2% | 20-30 min |
| 22-35°C (Extra Slow / Summer grade) | 1% | 15-25 min |
The Cure It system uses different catalyst grades (winter, summer, extra slow) for different seasons, not just different doses. If you're using Cure It, the kit comes with the right grade for the temperature band stated on the box. Mixing a Cure It winter catalyst with Cure It summer resin (or vice versa) voids the manufacturer guarantee.
Generic MEKP dosing (5 bands)
Most non-branded MEKP and Butanox M50 from suppliers like East Coast Fibreglass, MBFG, and CFS uses a single medium-reactivity formulation with five dose bands by temperature:
| Temperature | Generic MEKP dose | ml per kg of resin |
|---|---|---|
| 5-8°C | 4% | 40 ml/kg |
| 9-12°C | 3% | 30 ml/kg |
| 13-19°C | 2% | 20 ml/kg |
| 20-30°C | 1.5% | 15 ml/kg |
| 30-35°C | 1% | 10 ml/kg |
The generic system gives finer-grained control because the chemistry is the same across the temperature range. The Cure It table is simpler because each grade is pre-tuned to a temperature band.
Never go below 1% catalyst, regardless of how hot it is. Some forum posts mention 0.5% as a hot-day workaround. Below 1%, the reaction can't complete properly even if the laminate appears to set. The result is a partially cured surface that fails over time. The minimum 1% is a chemistry limit, not a marketing one.
Never go above 4% either. Above that, the laminate doesn't cure faster, it weakens. Free hardener floats around in the matrix causing soft spots and shrinkage cracks within weeks.
How to mix it correctly
Mixing MEKP into resin is the most safety-critical 90 seconds of any GRP job. Done wrong, you contaminate equipment, dose inaccurately, or trigger an early exotherm. Done right, it's straightforward.
The order of operations
Pour catalyst into resin, never resin into catalyst. The resin is the bigger volume and the safer environment. Pouring resin into a small bottle of catalyst risks splashing concentrated peroxide back at the operator and contaminates the catalyst container, which then can't be re-sealed for storage.
Pour the resin into a clean bucket first. Decant only what you'll use in the next 15-20 minutes. The standard working batch is 1-2 kg. Larger batches don't last long enough to be useful and the heat builds up in the bucket before you can spread it.
Stir the resin briefly with a flat-ended stick before adding catalyst. This redistributes any pigment that's settled and warms the surface slightly. Don't whip air into it, air bubbles in the catalysed resin become voids in the cured laminate.
Measure the catalyst with a calibrated 50ml medical syringe. A 50ml syringe with 1ml graduations costs around £2 – £2. Eyeballing the dose is the single biggest cause of GRP roof failures on DIY jobs. A "splash" might be 3% or 8%. The syringe means you know.
Add the catalyst to the resin in a steady stream, then stir for a full 60 seconds with the flat stick. Scrape the sides and bottom of the bucket. Uncatalysed streaks at the edges create localised uncured zones in the finished laminate even when the overall ratio is correct.
Apply within the working time for that temperature band. Once you've started, you can't pause for a tea break. If the phone rings, ignore it. If the catalysed resin starts going warm in the bucket before you've used it, get it out onto the roof immediately or pour water into the bucket and walk away.
Buy two cheap 50ml syringes. Use one exclusively for catalyst, the other for clean rinse water. Label them with permanent marker. Never use a catalyst-contaminated syringe for anything else, and never let a catalyst syringe touch any other resin additive (especially accelerator). Cross-contamination is how the worst MEKP accidents happen.
Working batch size
A typical 20m² extension roof needs roughly 1.5-1.7 kg/m² of resin for a saturated 450g/m² CSM laminate. That's around 30-34kg of resin total. A two-person team will mix that in 15-20 small batches over the course of the laminate stage, never carrying more than 1-2kg of catalysed resin at any time.
The reason for small batches isn't speed. It's containment. If something goes wrong with one mix (wrong dose, sudden weather change, dropped bucket) you've lost 1kg of resin, not 30kg. And the smaller the bucket, the more readily heat dissipates if something starts to exotherm.
What goes wrong
The two failure symptoms, sticky roof and bucket fire, both have multiple distinct causes. Diagnosing the right cause matters because the remedies are completely different.
"My roof is still sticky after three days"
There are three separate mechanisms that produce a tacky GRP surface. Two of them are recoverable. One isn't.
Cause 1, Under-catalysed laminate. You mixed too little catalyst (or measured by eye), or the temperature dropped during cure (a cold night after a warm afternoon application), or the catalyst was past its 6-month shelf life and had lost activity. The laminate cured partially. Surface still tacky, mat fibres visible through the resin, soft underfoot.
Recovery: if it's been less than 48 hours, more time and warmth will sometimes complete the cure. If it's been longer, sand back to bare mat, re-apply correctly catalysed resin, and continue.
Cause 2, Cold weather application. Below 5°C, standard MEKP-catalysed resin won't cure reliably even at 4% dose. Below freezing, it won't cure at all. The surface stays liquid then gels into a soft, semi-cured state that never hardens fully.
Recovery: sand back, wait for warmer weather, re-apply. Or use specialist "Extra Cold" formulations like CrysticROOF COOLCure (which uses a different catalyst, HBO 50, not standard MEKP).
Cause 3, Paraffin wax in topcoat melting in heat. GRP topcoat contains a small amount of paraffin wax that floats to the surface during cure to seal off oxygen (oxygen inhibits polyester cure at the air interface). On very hot days, particularly with a dark topcoat in direct sun, the deck surface can hit 50-60°C and the wax simply melts off the surface before it has done its job. The result is a permanently tacky topcoat that no amount of waiting will fix.
Recovery: there isn't one for the existing layer. Sand back the topcoat and reapply in cooler conditions, ideally early morning or late afternoon, with the deck in shade or overcast weather.
The three causes look identical to the homeowner. The difference is in the timing and the temperature history. If the laminate was sticky from the start and stayed sticky in cold weather, it's an under-cure issue. If the topcoat was fine for the first day and went tacky on a hot afternoon, it's wax migration.
"The bucket caught fire"
This is rarer but more dangerous. There are two distinct scenarios.
Scenario 1, Over-catalysed resin in a bucket. Above 4% catalyst, or at any catalyst level if a large volume sits in a bucket too long, the cure exotherm runs away. The resin starts to smoke (visible white vapour rising from the bucket), then darkens, then ignites. The fire is fed by the resin's own monomer (styrene) and burns hot.
Response: pour water into the bucket immediately and move it away from anything flammable. Water is the correct extinguisher for catalysed polyester resin and for MEKP itself. Do not use dry powder. Dry powder will work on the resin fire but not on residual MEKP, which can re-ignite. Water cools the bucket and floods the reaction.
Scenario 2, MEKP contacting an accelerator. This is the dangerous one. Pre-promoted polyester resins (the kind sold for roofing) already contain a cobalt naphthenate accelerator pre-blended into the resin at the factory, in tiny quantities (around 0.1-0.5%). That accelerator is what allows MEKP to work effectively at room temperature. The resin is safe to handle.
The fire risk arises when MEKP contacts pure, undiluted accelerator. This usually happens through container contamination. A measuring syringe used for accelerator on a previous job, a bucket that held accelerator, or a bottle stored next to MEKP that has leaked. Direct contact between MEKP and concentrated cobalt naphthenate or dimethylaniline can cause violent decomposition with potential explosion.
Response: prevention only. There's no "putting it out" of an MEKP/accelerator reaction. You stop it from happening by never bringing the two chemicals into the same workspace, by never reusing accelerator-contaminated equipment, and by storing them in different cabinets in different parts of the workshop or shed.
The practical fire risk in domestic GRP work is contaminating catalyst containers or syringes with accelerator residue, NOT the catalyst reacting with the polyester resin you bought. Modern roofing resins are pre-promoted at the factory with a tiny safe dose of accelerator. Adding extra accelerator yourself, or letting the catalyst touch a bottle that previously held accelerator, is what causes the fires you read about in incident reports.
Cost and where to buy
MEKP is sold by volume (in ml or L) or by weight (in g or kg). Roughly equivalent because the density is close to 1 g/ml. Most UK fibreglass suppliers ship in childproof vented bottles in three common DIY sizes.
A 250ml bottle costs £4 – £5 and is enough for around 12-25kg of resin depending on temperature and dose. That's enough for a small flat roof of 8-15m².
A 500ml/500g bottle runs £6 – £10 and covers a typical 20m² extension roof with some left over. Most DIY users buy this size.
A 1L/1kg bottle is £11 – £15 and is more catalyst than most domestic jobs need. Worth buying only if you're doing multiple roofs back-to-back within the 6-month shelf life.
A calibrated 50ml medical-grade syringe for measuring is £2 – £2. Buy two. The cost of a syringe versus the cost of redoing a roof because you over-catalysed by guessing makes this the highest-value safety purchase on any GRP job.
The major UK suppliers are MBFG (Manchester Bristol Fibreglass), East Coast Fibreglass Supplies (ECF), CFS Fibreglass, GRP UK Ltd (Stronghold), and Roofing Kits Direct. Prices are similar between them, with ECF and CFS slightly higher than MBFG for branded Butanox M50. Cure It catalyst is generally only sold as part of a Cure It system kit, not standalone.
Don't buy MEKP from generic Amazon or eBay sellers. Concentration and shelf-life integrity matter, and counterfeit MEKP at the wrong concentration is documented in industry warnings. Buy from named UK fibreglass suppliers who specify the formulation (Butanox M50, Trigonox 44B, or named generic) and the manufacturing date.
Storage
MEKP storage is the boring part of the job that prevents the dramatic part of the job. The rules are short and absolute.
Cool, dry, dark, below 25°C. Heat accelerates the slow decomposition of MEKP over time. Sunlight does the same. A garden shed in summer routinely hits 35-40°C inside. That's not acceptable for MEKP storage. A cool internal cupboard or a north-facing outbuilding is fine.
Away from accelerators, reducing agents, and acids. Cobalt naphthenate, dimethylaniline, amines, ferrous compounds, and acidic chemicals all react with MEKP. The bottle of accelerator from your old fibreglass job is not allowed in the same cupboard as your new bottle of MEKP. If you keep both in stock, store them at opposite ends of the shed in separate sealed containers.
In its original vented bottle. MEKP slowly decomposes and releases small amounts of gas. The childproof caps that come on supplier bottles include a vent. Decanting MEKP into a non-vented container risks pressure build-up and bottle rupture. Don't transfer to old bottles or jam jars.
Away from absorbent materials. Rags, sawdust, and paper soaked in MEKP are a spontaneous ignition risk. The 1979 factory fire mentioned earlier started from MEKP-soaked rags. Catalyst spills should be diluted with large volumes of water on a hard surface, not mopped up with absorbent material.
Within the 6-month shelf life. MEKP loses activity over time even in correct storage. After 6 months, the catalyst that should give you a 2% cure starts behaving like 1.5%, then 1%, then doesn't cure properly at all. The bottle date stamp is not optional information. If you find an old bottle of MEKP in the shed from a previous project, don't use it, buy fresh stock.
Never store MEKP in a fridge with food or drink. Some industrial chemists do refrigerate MEKP to extend shelf life, but this requires a dedicated chemicals fridge with no food contamination risk and lockable access. For a home user, an internal cool cupboard is the right answer. A drinks fridge in the kitchen is not.
PPE and emergency procedures
Treat MEKP like a strong acid. The hazards are similar even though the chemistry is different.
Eye protection. Wraparound safety goggles or a full face shield. Splash-rated, not generic safety glasses. Eye damage from MEKP is severe and permanent if not flushed within minutes.
Skin protection. Chemical-resistant nitrile or neoprene gloves, full sleeves, no exposed skin on forearms. Latex gloves are not adequate, MEKP penetrates them. Barrier creams on hands and forearms add a secondary layer.
Respiratory protection. For domestic quantities outdoors, ambient ventilation is usually enough. Indoors or in still air, an organic vapour respirator (FFP3 with ABEK filter cartridges, not a dust mask) is appropriate.
No food, drink, or smoking anywhere near the workspace. Smoking near MEKP is a fire risk. Eating or drinking risks accidental ingestion.
If MEKP gets in your eye
Irrigate immediately with running water for at least 15 minutes. Hold the eyelid open. Then attend A&E without delay, even if the eye looks fine. Damage from peroxide burns can develop hours after exposure.
If MEKP gets on skin
Remove contaminated clothing. Rinse the area with running water for 15 minutes. Do not use solvents to clean the skin (no acetone, no white spirit). If burning persists or the skin shows redness, attend A&E.
If MEKP is spilled on the ground
Dilute with large quantities of water. Do not attempt to mop up with rags, paper, or sawdust, those become spontaneous ignition hazards. For larger spills (more than a few hundred ml), use a manufactured catalyst spill kit which contains an inorganic absorbent.
If a bucket of catalysed resin starts smoking
Pour water into the bucket from a safe distance. Move the bucket away from anything flammable. Do not throw it in a bin or skip, the reaction can continue and ignite other waste. Leave the bucket outside on a hard surface until the resin has fully cured (typically a few hours), then dispose of it as cured plastic waste.
Disposal
Three categories of waste come off a GRP job. Each has a different disposal route.
Unused MEKP, keep in original bottle for the next job, within shelf life. If genuinely surplus past shelf life, take to a council household waste recycling centre and declare it as hazardous chemical waste. Don't pour it down the drain, and don't put it in general waste.
Cured catalysed resin, once fully hardened, polyester resin is essentially inert plastic. Dispose of as construction waste (skip or council household waste). Roofers often pour leftover catalysed resin into a sacrificial container, let it cure into a solid block, and bin it.
Empty MEKP bottles, rinse with water, allow to air-dry, then dispose with rigid plastic recycling. Do not crush or seal an unrinsed bottle (residual MEKP plus bottle compression risks decomposition).
Liquid catalysed resin that hasn't started to gel can be poured into water in a sacrificial bucket and left to cure. Once cured, it's safe waste. Pouring catalysed liquid resin straight to drain is illegal and damages drainage. Pouring acetone wash water down the drain is also not permitted.
Where you'll need this
- Roof covering, MEKP is required for any GRP fibreglass flat roof installation, mixed into both the laminating resin and the topcoat in small batches throughout the application
MEKP catalyst comes up on any extension or renovation project that involves a GRP flat roof, GRP repairs, or GRP rooflight upstands. It's not specific to kitchen extensions, the chemistry and procedures are identical whether you're doing a small porch roof or a 50m² rear extension.
Common mistakes
Eyeballing the dose. "It looked about right" is the most common cause of failed roofs and smoking buckets. Use a calibrated syringe every single time. The whole syringe costs less than two pints.
Using old catalyst. Catalyst that's been sat in the shed for a year doesn't trigger the same cure as fresh stock. Check the date stamp. If in doubt, buy a new bottle. A fresh bottle is cheap; a failed roof is not.
Wrong grade for the season. Cure It and CrysticROOF sell separate winter and summer catalyst grades. Using a summer grade in October produces under-cured laminate even at 4% dose. Match the grade to the season the kit was sold for.
Pouring resin into the catalyst bottle. Always the other way round. Catalyst into resin, in a bucket. Never resin into a catalyst bottle.
Mixing too much at once. A 5kg bucket of catalysed resin is a fire waiting to happen if it sits more than a few minutes. 1-2kg batches mixed and applied straight away. The "I'll do the whole roof in one mix" instinct kills jobs.
Storing catalyst next to accelerator. The most dangerous storage error. Pre-promoted resin already contains accelerator at safe levels, you almost never need to handle pure accelerator. If you do (some specialist systems use separate accelerator), keep it in a different cabinet entirely from the MEKP.
Confusing MEKP with MEK. MEK is a solvent for cleaning brushes. MEKP is a peroxide catalyst that can ignite spontaneously. Different bottles, different chemistry, vastly different hazards. Don't substitute one for the other and don't store them in the same place where labels could be confused.
Skipping PPE because "it's only one mix". Most MEKP injuries happen on the last bucket of the day when the operator is tired and rushed and decides not to bother with the goggles. The chemistry doesn't care that you're nearly finished.