Start by Identifying the Hull and the Damage

Before you mix a single drop of resin, confirm what your kayak is actually made from. Rotomoulded polyethylene hulls — the tough, slightly waxy plastic used on most recreational sit-on-tops — will not accept a fibreglass patch. Resin simply will not key to them, and the repair will peel off within a season. Those hulls need plastic welding instead. If your boat is GRP, composite, or a glass-reinforced layup with a gelcoat finish, you are in the right place.

Next, assess the damage honestly. Light scratches that only cut into the gelcoat need nothing more than a fill and polish. A proper repair is required when you can see frayed glass fibres, when the laminate flexes under thumb pressure, or when a crack runs deeper than a millimetre or two. Tap around the area with a coin — a dull, flat thud rather than a sharp ring suggests delamination hiding beneath the surface. Grind that out too, or the patch will sit on a foundation of dead laminate.

Finally, make sure the hull is dry. Composite soaks up water through any breach, and trapped moisture will turn to steam under an exothermic cure, blowing pinholes through your carefully laid work. Leave the boat somewhere warm and ventilated for at least a week, longer if the damage has been exposed for a while.

Gather the Right Materials

A fibreglass repair is only as good as its consumables, so buy properly rather than raiding the back of the shed. You will need:

• Chopped strand mat (around 300–450 gsm) for building thickness, plus a lighter 200 gsm mat or a fine woven cloth for the surface layer.
• Polyester resin with MEKP catalyst, or epoxy if you want the strongest bond and are prepared to pay for it. Epoxy adheres better to awkward substrates and shrinks less, but polyester is perfectly adequate for most hull repairs and far cheaper.
• Acetone for degreasing and tool cleaning.
• Peel ply and release film, plus a flexible clamping sheet.
• Mixing cups, graduated syringes, a stiff-bristled stippling brush, a consolidation roller, and a squeegee.
• Nitrile gloves, safety glasses, and a proper organic-vapour respirator. This is not optional — styrene and amine hardeners are genuinely nasty.

Work outdoors or in a well-ventilated garage, and keep the ambient temperature between 15°C and 25°C. Cold resin refuses to cure properly; hot resin gels before you can lay a second layer.

Preparing the Hull Surface

Preparation is roughly eighty per cent of the job. Start by removing all traces of wax, polish, and antifoul from a generous area around the damage using acetone and clean rags, changing the rag often. Then grind back the damaged laminate with a 40 to 60 grit disc on an angle grinder or a coarse sanding block, working until you reach sound, dry, greyish glass.

Crucially, taper the edges. Feather the repair outwards at a ratio of roughly 1:10, so a 3 mm deep gouge is bevelled back about 30 mm in every direction. This gives each successive layer of glass something to bond to and avoids a hard, stress-concentrating step at the patch edge. Wipe the whole area down with acetone once more and let it flash off completely before you mix anything. Mask off the surrounding gelcoat with tape if you want a tidy edge.

Cutting the Glass to Size

Make a paper template of the repair zone, then cut your patches on a clean board using sharp scissors. Aim for the first layer to sit just inside the bevel, with each subsequent layer slightly larger, so the outermost patch overlaps the original laminate by at least 40–50 mm all round. Three or four layers is usually right for a hull repair — enough to restore stiffness without adding unnecessary weight.

Handle chopped strand mat gently; it is held together by a light binder and frays easily. Cut it dry, stack the pieces in the order you plan to lay them, and keep them somewhere dust-free. If you are using woven cloth for the top layer, cut it on the bias so it drapes into curves rather than bridging them.

Wetting Out and Clamping

Mix your resin in small batches — 200 ml at a time is plenty. For polyester, add catalyst at around 2 per cent by volume (roughly 4 ml per 200 ml) at 20°C, adjusting down slightly in warm weather. Epoxy must be measured by weight to the manufacturer's ratio, so use scales.

Brush a thin coat of catalysed resin onto the prepared hull, then lay the smallest patch and stipple it firmly with a dry brush, working from the centre outwards to drive resin through the fibres and push air ahead of you. Add the next layer while the first is still wet, then the next, stippling each one down. Roll over the whole patch with a consolidation roller to squeeze out excess resin and any trapped bubbles. You want a translucent, slightly glossy laminate — not a dry, whitish one, and not a swimming pool of resin.

Finish with a layer of peel ply, then a flexible sheet and either weights, spring clamps, or a vacuum bag if you have one. On a curved hull, gentle, even pressure beats brute force — over-clamping starves the laminate of resin.

Curing, Sanding and Gelcoat

Leave the patch undisturbed until it has fully hardened. Polyester will gel in twenty to forty minutes but needs a full 24 hours at room temperature before you sand it; epoxy wants longer still. Once cured, peel off the release film and trim any rough edges with a sharp blade while the resin is still slightly green, which is far easier than grinding later.

Sand the repair flush with 80 grit, then refine through 120 and 240, feathering the edges into the surrounding gelcoat so you cannot feel a ridge with your fingertips. Wash the dust off with warm soapy water and let it dry.

For the final watertight finish, apply a two-part gelcoat or a pigmented topcoat matched to your hull. Mix in the wax additive if you are brushing it on in the open air, or cover it with release film if you are not. Build two or three thin coats rather than one thick one, then once cured, wet-sand progressively through 600, 800 and 1200 grit, finishing with a cutting compound and polish. Done patiently, the repair will be invisible from a metre away and will outlast the rest of the hull.