Why is my gelcoat still sticky after curing, and how can I fix it?
I repaired a few chips in my cockpit with white gelcoat yesterday, but the surface is still sticky and clogs the sandpaper. How can I tell whether it needs more curing time, an air barrier or scraping out and starting again?
Answer
Gelcoat usually stays sticky because air has inhibited the surface cure, or because incorrect catalyst dosing, poor mixing or low temperature has prevented the repair from curing throughout. Check beneath the sticky skin first: a hard repair may need an air barrier, while soft gelcoat usually needs removing and replacing.
Check beneath the surface
These checks apply to conventional polyester gelcoat mixed with MEKP catalyst. Epoxy repair coatings and specialist gelcoat systems have different mixing and curing requirements.
Read the tin first. Ordinary unwaxed gelcoat is designed to cure against a mould or remain tacky between layers. Exposed to air, its surface can stay sticky even when the material underneath is sound. Waxed gelcoat, often sold as topcoat or flowcoat, contains an additive that rises to the surface and excludes air.
At a sacrificial edge, use a clean scraper to check beneath the surface:
- Hard underneath, with a thin sticky skin: oxygen inhibition is likely, particularly with unwaxed gelcoat.
- Soft or rubbery beneath the skin: suspect incomplete curing throughout the repair. Material that scoops out easily is another sign.
- Alternating hard and soft patches: suspect uneven catalyst mixing.
- Firm with a greasy surface: this may be wax residue rather than uncured resin.
Do not judge by touch alone or keep clogging fresh sandpaper with sticky material. Once fully cured and cleared of surface residue, gelcoat should be hard enough to produce sanding dust.
Check the curing conditions
Measure the hull temperature as well as the air temperature. A boat chilled overnight can remain cold long after the workshop warms up. Many repair products work best around 18 to 25°C. Minimum application temperatures and cure times vary, so follow the product sheet.
If the mix was correct but the repair has been cold, allow the specified curing period at a steady, suitable temperature before deciding it has failed. Warm the surrounding space gently and evenly. Do not blast the patch with a heat gun.
Many polyester gelcoats use approximately 1 to 2% MEKP by weight, but the permitted range depends on the resin, catalyst grade and temperature. At 2%, 100 g of gelcoat needs 2 g of catalyst. Do not substitute a guessed number of drops. Use suitable measuring equipment and mix thoroughly, scraping the pot's sides and bottom without whipping in air.
Too little catalyst, material missed during mixing, deteriorated stock or unauthorised solvent additions can all cause curing problems. Excess catalyst can overheat or discolour the repair and make it brittle. It is not a reliable shortcut.