A GRP boat is made of glass fiber reinforced plastic. Glass fibers are combined with a synthetic resin system to form a lightweight, stable and corrosion-resistant laminate that has been used in boatbuilding for many years.

Depending on the construction, a boat may be based on polyester resin or epoxy resin. This difference is important for repairs because not every resin system adheres equally well to every substrate. A professional boat repair therefore always starts with assessing the laminate, the damaged area and the later area of use.

When repairing a boat, the resin system is selected according to the existing laminate, the type of damage and the expected load. As a basic rule, epoxy resin should also be used on an epoxy laminate because polyester resin generally does not achieve particularly good adhesion on epoxy resin.

Epoxy resin, on the other hand, adheres very well to both polyester laminate and epoxy laminate if the substrate is sanded, clean, dry and free of grease. Polyester resin adheres best to polyester laminate and is therefore mainly suitable for classic repairs on polyester GRP boats.

You can find more basics in the lexicon articles Epoxy Resin – Properties, Benefits and Applications and Epoxy Resin vs. Polyester Resin – Material Comparison.

Osmosis is a typical type of damage in GRP boats that remain in the water for long periods. Water diffuses into the laminate. In combination with components of the polyester laminate, reactions can occur that cause blisters in the laminate or gelcoat layer.

Osmosis often appears as blisters that may contain a yellowish-brown liquid. Boats that stay permanently in the water are at higher risk than boats stored dry or out of the water. Modern high-quality polyester resins and improved processing have significantly reduced osmosis damage, but epoxy resin usually remains the first choice for osmosis repairs.

For a stable boat repair, you need a suitable resin system, suitable reinforcing fibers, sanding and cleaning tools as well as filling and sealing materials. The selection depends on the type of damage, substrate and area of use.

Before repair, it must be clarified whether the issue is superficial gelcoat damage, localized damage, a crack, delamination or a larger laminate defect. Osmosis blisters and moisture in the laminate must also be treated differently from a small impact mark.

Localized or Large-Area Repair?

• Localized repair: A single damaged area is ground out, filled and rebuilt with reinforcing fiber and resin.
• Large-area repair: A larger area is sanded, cleaned and over-laminated with one or more layers of reinforcing fiber.
• Osmosis repair: Affected areas are opened, dried, filled, laminated and sealed with a suitable system.

Preparation determines the adhesion of the repair. First, damaged areas are cleaned, loose or damaged parts are removed and the repair zone is sanded generously. The substrate must be dry, stable, dust-free and free of grease.

How to Prepare the Repair Area

1. Remove damaged gelcoat or laminate areas.
2. Grind cracks and broken-out areas into a V-shape.
3. Sand the repair area with sufficient overlap.
4. Thoroughly vacuum dust and clean the surface.
5. Allow moist areas to dry completely.
6. Select the suitable resin system and reinforcing fiber.

Broken-out areas, depressions and ground-out zones are filled before laminating. Below the waterline, simple polyester filler should not be used because polyester filler can be hygroscopic and may absorb moisture.

Suitable options include filled epoxy resins, for example with glass microspheres, chopped glass fibers or cotton fibers, or a high-quality epoxy filler. This allows uneven areas to be leveled and stable transitions to be created for the laminate build-up.

For GRP and boat repairs above and below the waterline, Epoxy Filler Nautic Ultra-Light is suitable, for example.

After filling, the damaged area or prepared section is laminated. Reinforcing fibers are saturated with resin and applied to the prepared substrate. It is essential that the fibers are completely wetted and that no air bubbles remain in the laminate.

Which Reinforcing Fiber is Suitable?

When using epoxy resin, glass filament fabric is recommended. With polyester resin, both glass fiber mats and glass filament fabric can be used. Glass fiber mats are held together with a binder that dissolves through the styrene in polyester resin. This is why classic glass fiber mats are particularly suitable for polyester resin.

After laminating, the laminate must cure completely. The area is then sanded, filled if necessary and sealed with a suitable top layer. Depending on the build-up, this can be done with topcoat, epoxy filler and paint or a suitable coating system.

Topcoat serves as a finishing layer and protects the laminate. For repairs in visible areas, a clean sanding and paint build-up is also important so that the repair visually matches the existing boat surface.

You can find more information on finishing layers in the articles Topcoat – useful information and Difference between Gelcoat and Topcoat.

The most common mistakes in boat repairs are caused by incorrect material selection, insufficient preparation or processing too quickly. Especially below the waterline, repairs should be carefully planned and carried out with suitable materials.

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