Short answer
A hole or crack through the laminate is repaired with fibreglass cloth and epoxy. Grind away all damaged material, grind a long bevel around the hole, put a backing on the inside and build up new layers of glass. Then fill, sand and paint or apply gelcoat. If the damage is in the keel, transom, at through-hulls or over a large area, have a boatyard assess it.
Only surface damage in the gelcoat? Read repairing gelcoat instead.
Assess the damage first
| What you see | What it usually is | DIY? |
|---|---|---|
| Crack only in the gelcoat, laminate hard | Cosmetic damage | Yes, see repairing gelcoat |
| Star or spider-web cracks near a bulkhead, window or fitting | The hull flexes there | Yes, but reinforce the cause |
| Visible glass fibre, a hole or a crack through the laminate | Structural damage | Yes, if the area is small and not load-bearing |
| Soft, spongy or dull sound when you tap | Delamination or wet core | See soft deck and wet core |
| Damage to keel, keel bolts, transom or engine bed | Structural parts | Boatyard |
Tap around the damage with a small hammer. West System points out that the laminate may be damaged some way beyond what you can see. Where the sound turns dull, the laminate is not sound.
Star cracks: cosmetic or structural?
According to West System, cracks that appear gradually and grow are usually a sign that the panel is flexing. They are often found at a bulkhead, at the joint between deck and coachroof, or around windows. If you only fill the crack, it will come back.
- Around a fitting (cleat, bollard, stanchion): the fitting loads too thin an area. Remove it, repair, and refit it with a larger washer or a backing plate underneath.
- In the middle of a large panel: the panel is too flexible. It may need reinforcing with extra layers of glass fibre on the inside.
- After an impact: check the inside. If there are cracks there too, the damage goes through the whole laminate.
Materials and tools
| You need | For |
|---|---|
| Epoxy with hardener (e.g. West System 105 with 205 or 206) | Laminating and filling |
| Fibreglass cloth or biaxial cloth | New layers in the hole |
| Epoxy thickener | Adhesive and filler |
| Angle grinder or random orbital sander, 50 and 80 grit sandpaper | Removing damaged material, grinding the bevel |
| Thin sheet of laminate, plastic or board | Backing on the inside of the hole |
| Filling knife, squeegee, disposable brushes, mixing cups | Working in the epoxy |
| Nitrile gloves, safety glasses, respirator with particle filter | Protection from epoxy and sanding dust |
West System 105 is mixed 5 parts resin to 1 part hardener by volume. With 205 (fast) it must be at least 4 °C, with 206 (slow) at least 16 °C. The pot life of 205 is 9–12 minutes at 22 °C. Mix small amounts. If you use another brand, follow its mixing ratio.
Step by step
- Dry. The laminate must be dry. If the boat has just been hauled out, wait and let it dry under cover.
- Remove the damage. Grind or cut away everything loose and cracked until you reach sound laminate. Keep the hole round or oval.
- Grind the bevel. West System specifies at least 12:1, meaning 12 mm of bevel per mm of laminate thickness. A 5 mm hull gets a 60 mm bevel all round. A long bevel gives a large bonding area.
- Support from inside. If you can reach the inside, tape or glue a thin panel behind the hole. If you can't, push a piece of laminate through the hole and bond it to the inside edge.
- Clean. Wipe with degreaser, sand everything with 80 grit to a matt finish and vacuum.
- Prepare the layers. Cut cloth in several sizes. The smallest covers the hole itself, the largest reaches the outer edge of the bevel. Together the layers should be slightly thinner than the surrounding laminate.
- Laminate. Brush thin epoxy onto the bevel, then thickened epoxy. Lay the cloth from smallest to largest and work the epoxy in with a squeegee until the cloth is transparent with no air bubbles.
- Let it cure. Cured epoxy can develop a waxy film (amine blush). Wash it off with water and an abrasive pad before sanding, not with solvent.
- Fill and sand. Fill uneven areas with thickened epoxy. Sand with 50 and then 80 grit. Finish with 2–3 thin coats of epoxy and wet sand with 180.
- Finish. Below the waterline: epoxy primer and antifouling. Above: paint, or gelcoat as directed by the epoxy manufacturer.
Common mistakes
- Short bevel. The repair then lets go at the edge.
- Polyester as adhesive on old, cured laminate. Epoxy bonds better, see polyester or epoxy
- Repairing damp laminate. The water gets trapped.
- Too cold. Epoxy won't cure below the hardener's minimum temperature.
- Filling star cracks without fixing why the hull flexes.
Safety
Epoxy can cause allergies. According to the Swedish Work Environment Authority, the allergy can develop after a single splash. Use nitrile gloves and safety glasses, and wash skin with soap and water, never with solvent. Do not breathe in sanding dust from fibreglass and epoxy: use a respirator with a particle filter and vacuum while you sand. If you get a rash, stop working with epoxy.
When to hire a boatyard
Damage to the keel, keel attachment, rudder heel, transom or engine bed is structural. The same goes for holes after running aground and damage larger than the palm of a hand below the waterline. Get a quote from a boatyard near you via Marinas & boatyards.
Frequently asked questions
Can I use chopped strand mat with epoxy?
Standard chopped strand mat is made for polyester and held together by a binder. With epoxy most people use woven cloth or biaxial cloth, which is stronger for the same weight.
How wide a bevel should I grind?
West System specifies at least 12:1. A 5 mm laminate therefore needs about 60 mm of bevel around the whole hole.
Can I do the repair with the boat in the water?
No. The laminate must be dry and the epoxy needs the right temperature to cure. Haul the boat out.
How do I repair old screw holes?
Drill the hole out slightly, fill with thickened epoxy and redrill once it has cured. If the laminate around the hole is soft, read about moisture in the core.