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.