How to Repair Fibreglass Damage on a Small GRP Hull

A step-by-step, practical guide for small-boat owners: from minor gelcoat chips to larger fibreglass repairs. Clear instructions, materials, and safety tips so you can get back on the water.

Overview

GRP (glass-reinforced plastic) is durable, but chips, cracks and small holes happen. Repairs range from cosmetic gelcoat touch-ups to structural glass/laminate fixes. This guide covers common damage types, materials to choose, and a staged repair process so you can match strength and finish correctly.

When to tackle it yourself

Do it yourself: Small gelcoat chips, surface scratches, hairline cracks, and localized delamination where the laminate is intact.
Call a pro: Major structural damage (large holes, crushed hull sections, severe delamination across large areas, or keel/engine mounts if load-bearing).

Safety first

Always: Wear a respirator suitable for organic vapours and dust (P2/P3 for dust plus organic cartridge for solvents), nitrile gloves, eye protection and ear protection when grinding. Work in a well-ventilated area and keep dust/swarf away from skin.

Materials & tools

Resins & consumables

Polyester resin + MEKP catalyst (common, cheap) or epoxy resin (better adhesion and moisture tolerance).
Chopped strand mat (CSM) and woven roving (for strength where needed).
Gelcoat (colour-matched) or gelcoat repair paste.
Fairing compound (polyester or epoxy fairing filler) and finishing putty.
Sandpaper: 80, 120, 240, 400, 800, 1500 grit.
Isopropyl alcohol or acetone for cleaning.

Tools

Grinder with fibre disc or coarse sanding pad.
Dremel or small router (for precise beveling).
Brushes for resin, rollers and plastic spreaders.
Mixing cups, sticks and scale (for accurate resin/catalyst mix).
Clamps, plastic film, release film or peel‑ply for lamination.
Heat lamp or infrared heater (optional; speeds cure in cool conditions).

Understand the damage type

Inspect the area carefully. Common situations include:

Damage	Description	Repair approach
Gelcoat chip/scratch	Shallow damage limited to the gelcoat surface	Clean, fill with gelcoat or paste, sand & polish
Hairline crack	Thin cracks, often only in gelcoat	Open slightly, fill with resin/gelcoat, reinstate finish
Through-hole / small puncture	Hole through laminate	Patch from inside with woven mat and resin, fair & finish outside
Delamination	Laminate separated from core (if cored hull) or local laminate separation	Assess extent; if small, inject resin or replace laminate; if large, professional repair

Step-by-step repair: small gelcoat chip or scratch

1 — Clean the area: Wash with fresh water, remove grease, then wipe with acetone or isopropyl alcohol. Let dry.

2 — Sand lightly: Sand around the chip with 240 grit to remove loose edges and feather the gelcoat. For deeper chips, open to sound material but don’t cut away good laminate.

3 — Fill with gelcoat: Use colour-matched gelcoat paste or mix gelcoat with pigment. Apply in thin layers, allowing tack-off between layers per manufacturer instructions until slightly above surface level.

4 — Sand & polish: Start with 240/400 grit to level, then 800/1500 grit wet sanding. Finish with rubbing compound and polish to match surrounding gloss.

Step-by-step repair: small hole or through-laminate patch (structural)

Note: For holes or damaged laminate, you’ll usually patch from the inside where possible to restore full strength.

1 — Expose & prepare: From outside, grind back to solid laminate and bevel the edges to create a square or oval patch area. From inside, clear access and remove any loose material.

2 — Dry-fit glass layers: Cut layers of CSM or woven cloth to progressively larger sizes (e.g., 1 layer 50mm larger than hole, next 100mm larger, etc.).

3 — Layup: Mix resin (polyester or epoxy) with accurate catalyst amounts. Brush a tack coat, then lay the smallest patch first and impregnate with resin using a roller ensuring no dry spots or air pockets. Repeat with larger patches until laminate thickness matches original.

4 — Cure: Allow full cure as per resin instructions. Heat lamps can speed cure but avoid overheating (follow safety and resin temp guidance).

5 — Grind & fair: Trim excess, grind flush, and feather edges. Apply fairing compound to level and sand smooth.

6 — Topcoat: Apply gelcoat or topcoat paint to match. For best results use a colour-matched gelcoat or polyurethane marine paint over a suitable primer.

Tips for a stronger, longer-lasting repair

Where strength matters, use woven roving or biaxial cloth in the inner layers — these fabrics give better tensile strength than chopped strand mat alone.
Epoxy resin bonds better to GRP and is less prone to blistering if moisture is present, but it doesn’t always accept polyester gelcoat — you may need an epoxy primer before painting.
Always follow the resin maker’s mix ratios and working temperatures — undercatalysing causes soft patches, overcatalysing causes excessive exotherm and poor cure.
Work in shallow layers to avoid excessive heat build-up which can distort surrounding laminate.

Common mistakes to avoid

Rushing sanding before the laminate is fully cured.
Using polyester resin over an epoxy-only surface without proper adhesion prep.
Leaving water or oil contamination — always clean thoroughly before laminating.
Failing to match laminate directionality on structural repairs (woven/biaxial orientation).

Estimated times & curing guide

These are general timings — always check product data sheets:

Light gelcoat tack-off: 15–30 minutes (thin layers)
Polyester laminate initial cure: 30–60 minutes (touch), full cure 24 hours
Epoxy: longer working time; full mechanical strength often 24–72 hours depending on temperature

When to call a professional

Seek professional repairs if the damage affects structural integrity, near load-bearing fittings (engine mounts, keel), or if you find extensive core rot or widespread delamination. Professionals have ovens, vacuum-bagging and gelcoat matching tools that give superior results.

How to Repair Fibreglass Damage on a Small GRP Hull