Fibreglass & Carbon Fibre Repair
Identify the resin, prepare the surface, and patch gelcoat chips, cracks, blisters, and carbon spars onboard.
IDENTIFY · GRIND · DRY · LAY · CURE · FAIR
A durable composite repair matches the resin to the original laminate, tapers the scarf, dries the surface, and wets the cloth fully. Epoxy is the universal repair resin; polyester only bonds to polyester.
Step 1 — Identify the Resin
Epoxy
- Smell
- Mild, slightly sweet chemical odour when uncured.
- Look
- Clear to amber; very hard, glossy surface. Bonds well to almost anything.
- Used in
- Modern hulls, carbon fibre, spars, structural repairs, and as a repair resin over polyester.
- Compatibility
- Bonds to polyester, vinylester, epoxy, wood, and metal. The safest universal repair resin.
Polyester (Ortho/Iso)
- Smell
- Strong, sharp styrene odour when uncured or warmed.
- Look
- Usually pigmented gelcoat or white/green tint; slightly softer than epoxy.
- Used in
- Older production fibreglass hulls, deck mouldings, and gelcoat.
- Compatibility
- Does NOT bond well to epoxy. Best repaired with polyester or vinylester, or overcoated with epoxy.
Vinylester
- Smell
- Styrene odour, similar to polyester but slightly sharper.
- Look
- Clear to pale yellow; tougher and more flexible than polyester.
- Used in
- Blister-resistant hull skins, fuel tanks, and high-performance laminates.
- Compatibility
- Bonds to polyester and itself well; epoxy can bond to it but test first.
How to Test Which Resin You Have
Solvent (Acetone) Test
How: Dab a few drops of acetone on a hidden scrap or the back of a sanding dust sample.
Result: Polyester softens, goes tacky, or dissolves. Epoxy is unaffected — stays hard and glossy. Vinylester is mostly resistant but may dull slightly.
Burn Test
How: Take a tiny shaving of resin (no gelcoat), hold in tweezers, and ignite. Observe smoke and smell.
Result: Polyester: black, sooty smoke with strong styrene smell. Epoxy: pale smoke, faint sweet/chemical smell, burns briefly. Vinylester: sooty but less styrene smell than polyester.
Visual & Touch Test
How: Inspect the laminate surface, sanding dust, and any exposed cloth weave.
Result: Epoxy is hard and glassy with clear amber tone. Polyester is often pigmented or gelcoated and slightly softer. Carbon fibre is almost always epoxy-laminated (black, glossy, hard).
Tools & Materials
Fibreglass Repair Procedure
Identify the Resin
Run the solvent and visual tests above. When in doubt, repair with epoxy — it bonds to polyester and vinylester safely. Never apply polyester over epoxy.
Assess the Damage
Gelcoat chip: cosmetic only. Crack: check if it penetrates the laminate. Blister (osmosis): tap with a coin — dull sound means delamination. Soft or flexing area: structural, seek surveyor.
Grind a Tapered Scarf
Sand the damaged area into a 12:1 scarf (12mm of taper for every 1mm of laminate thickness). Remove all loose, wet, or dark laminate. Clean dust with acetone.
Dry the Laminate
For blisters or wet areas, dry thoroughly — heat lamp or warm air for hours to days. Repairing wet laminate traps moisture and fails later.
Mix the Resin
Measure resin and hardener precisely by volume or weight as specified. Mix slowly for 2 full minutes, scraping the sides. Do not whip air in.
Wet-Out the Surface
Brush a thin resin coat onto the scarfed area. Apply CSM first (binder dissolves in polyester/vinylester only — use chopped strand compatible with epoxy or biaxial cloth for epoxy repairs).
Lay the Cloth
Lay woven roving or biaxial cloth into the wet resin, stippling with a brush to saturate and remove air. Add layers, each fully wetted, largest patch first then smaller on top.
Apply Peel Ply
Lay perforated peel ply over the final layer and stipple flat. This leaves a textured surface for the next coat and removes excess resin.
Cure
Let cure per the resin spec (often 24h at 20°C, longer if cold). Epoxy needs a minimum 15°C cure temp; warming accelerates. Do not sand until hard and tack-free.
Fair & Finish
Remove peel ply, sand the patch fair with the hull (80 → 120 → 220 grit). Fill pinholes with epoxy/filler, then apply gelcoat (polyester hull) or epoxy topcoat/paint.
Inspect & Test
Tap-test the repair — a clear ring indicates a sound bond; a dull thud means delamination. Check for pinholes and re-coat before antifouling.
Carbon Fibre Repair — Key Differences
Epoxy Only
Carbon fibre is always laminated with epoxy. Never use polyester or vinylester resin on carbon — the bond is poor and the spar will fail.
Match the Fibre Architecture
Match the original layup orientation (uni, biaxial, or twill). A mismatched patch creates stress concentrations and can crack the spar.
Vacuum Bag for Strength
Vacuum bagging removes excess resin and trapped air, maximising fibre-to-resin ratio. A hand-layup carbon patch is weaker than the original.
Cut & Sand Safely
Carbon dust is conductive and can short electronics. Wear a respirator, vacuum (do not blow) dust, and isolate the work area from instruments.
Scarf Heavily
Carbon spars need a 50:1 or greater scarf ratio for structural repairs. A short taper on a loaded spar will crack at the edge of the patch.
Cure Warm
Epoxy on carbon should be post-cured with gentle heat (40–60°C) for full mechanical properties if the repair is structural.
Safety & Curing Tips
Scarf (Taper) Diagram
Grind a 12:1 taper (12mm per 1mm thickness) into the original laminate, then lay matching cloth layers into the tapered area.