Fibreglass Roof Catalyst Guide: MEKP % by Temperature

The dose table

MEKP catalyst is dosed as a percentage of the combined weight of resin and topcoat you're mixing, set by the air temperature: 26–30°C → 1%. 18–25°C → 2%. 11–17°C → 3%. 5–10°C → 4%.

Two hard limits sit either side of that table. Never lay GRP below 5°C — the cure stalls and no amount of extra catalyst rescues it. And never exceed 4%: over-catalysed resin cures brittle, can exotherm hard enough to crack, and porous laminate is the result.

Work the numbers per mix, not per job. If you're laminating with 2 kg of resin in the bucket at 15°C, that's 2 kg × 3% = 60 g of catalyst. A dispenser or graduated cup pays for itself on the first day — MEKP is roughly 1 g per ml, so 60 g is about 60 ml.

Working time: what each dose feels like

At the correct dose you get roughly 15–25 minutes of working time in the bucket, and the laminate firms over the following hour. That's why GRP is applied in small mixes — 2–5 kg at a time — rather than catalysing a whole tub.

Temperature swings during the day matter more than people expect. A March morning at 8°C wants 4%; by 1pm the deck in full sun might be over 18°C and the same dose will gel your mix in minutes. Re-check the temperature and adjust every couple of hours.

The deck temperature can differ from air temperature — a dark deck in direct sun runs much hotter. If the resin is dragging or going stringy noticeably faster than the last mix, drop the dose a step before the next one.

Cold weather tactics

Between 5 and 10°C GRP is workable but unforgiving: 4% catalyst, small mixes, and store the resin somewhere warm overnight (a cold tub cures slower regardless of dose). Start late morning so the laminate gets the warmest hours to cure before evening damp.

Dew is the silent killer in the shoulder seasons. Moisture on or in the deck stops the resin bonding and shows up later as blisters. If the boards show any surface moisture, or rain is forecast before the topcoat can go tack-free, don't start.

Below 5°C, stop. There is no dose that fixes it — under-cured laminate stays soft and cheesy, never reaches strength, and usually has to come off. A lost day is cheaper than a lost roof.

Hot weather tactics

Above 26°C, drop to 1% and shrink your mixes. Keep the resin tub in the shade — resin at 35°C in the sun behaves like resin two doses hotter. Mix in a wide, shallow container rather than a deep bucket: a deep mass of catalysed resin traps its own heat and gels faster.

Start early. On a hot day the first mixes of the morning are the easy ones; by mid-afternoon on a dark deck you may be down to ten minutes of usable time. Plan the layout so you're never wetting out more mat than one mix can cover.

Above 30°C, seriously consider waiting for a cooler day. Flash-curing resin doesn't just waste material — it leaves dry, poorly-bonded joins between one mix's area and the next.

Handling catalyst safely

MEKP is a corrosive, oxidising liquid (UN3105) — it will chemically burn eyes on contact. Safety glasses and nitrile gloves every single time you dispense, no exceptions, and keep a bottle of clean water within reach for immediate flushing.

Never pour catalyst back into its container, never let it contact acetone in a confined mix, and never leave it in sunlight. Store it in its original vented bottle, cool and away from the resin. It ships on a specialist hazardous-goods network for good reason.

Add catalyst to resin, stir for a full minute scraping sides and bottom, and mix only what you'll use in the next 15 minutes. An unstirred pocket of neat catalyst gives you a brittle streak in the laminate; an unmixed pocket of resin never cures at all.

Frequently asked questions