Boat Gelcoat Repair: Fixing Blisters, Cracks, and Crazing


Understanding and Addressing Gelcoat Issues

Protective gelcoat is essential for fibreglass boats as it shields them from the elements and enhances their appearance. However, gelcoat can develop issues like blisters, cracks, and crazing over time, compromising aesthetics and structural integrity. Fortunately, this article provides detailed instructions on repairing gelcoat, covering common issues and outlining each step. 
If you own a boat or are new to DIY projects, this article can help you. It will provide techniques, materials and tips for successful gelcoat repair.

Continue reading to learn more. 

Gelcoat Blisters:

Blisters are raised bubble-like formations on the gelcoat surface caused by moisture intrusion into the fibreglass layers. Untreated moisture can worsen the damage, potentially compromising structural integrity.

Gelcoat Cracks:

Cracks can result from impacts, stress, or ageing. Collisions, handling, and natural ageing can cause visible cracks on the gelcoat surface.

Gelcoat Crazing:

Crazing is the formation of tiny cracks, resembling a spider web or broken glass, caused by UV exposure, temperature fluctuations, and chemical exposure. While often superficial, it can be unsightly and may lead to water absorption if ignored.

Understanding the causes of gelcoat blisters, cracks, and crazing is essential in determining the appropriate repair approach. By addressing these issues promptly and effectively, you can preserve the appearance and longevity of your boat's gelcoat surface.


How to Repair Damaged Gelcoat on a Boat

Marine boat repair

How to Repair Blisters in Gelcoat

Step 1: Dry and Stabilise the Area

Norton Marine boat repair sanding

The boat must be lifted out of the water and dry-docked during the repair.

It's a good idea to check the bottom of a boat right after taking it out of the water. This helps ensure we get the right information because blisters can shrink quickly, sometimes even in an hour.

It's important to note that blisters typically remain elevated even after shrinking compared to the surrounding surface.

Step 2: Remove the Damaged Gelcoat

To keep yourself safe, remember to wear safety equipment like gloves, goggles, a mask to help you breathe, and ear defenders to protect your ears when you use power tools.

Once the area is dry, you can use a scraper or a straight/mini angle grinder with a sanding attachment to remove the damaged gelcoat carefully.

The recommended discs for this process are Norton Blaze A995, Norton Mesh M920 or Norton Xtreme S Speedlock discs in coarse grit, which can be adjusted with the sanding machine for different levels of aggressiveness with various backup pads. 

If the repair is small, you can also opt for hand sanding using our R222 Handy Roll, which works well on curved surfaces.

Be cautious not to damage the underlying fibreglass layers while gently scraping away the blistered or raised portions. To achieve a smooth and level surface, gradually feather the edges of the surrounding gelcoat.

Step 3: Sand the Area

With the damaged gelcoat removed, MeshPower sanding discs M920 with a DA sander or MeshPower sheets M920 with a manual sanding block to smooth the surface further – start with 120 grit and progress to 240 grit to ensure the surface is as smooth as possible.

Sand the area in a circular motion to ensure a good bond between the new gelcoat and the fibreglass. Continue sanding until the surface feels smooth and any remaining rough edges are removed.
Step 4: Clean and Prepare the Surface

Thoroughly clean the repaired area to remove dust, debris, or residue from the sanding process.

Use a mild detergent and water solution or a gelcoat-specific cleaner.

Rinse the surface and allow it to dry completely. Once dry, use a solvent like acetone to remove any remaining contaminants and ensure optimal adhesion of the new gelcoat.
Step 5: Apply New Gelcoat

Thickened gelcoat, which includes colloidal silicas, is a good option for replacing the gelcoat and the first few layers of the underlying laminate. However, if the blisters have affected deeper layers, it's best to follow proper practice and replace the removed fibreglass reinforcement during the grinding process.

To prepare the gelcoat mixture, follow the manufacturer's instructions and combine the high-performance gelcoat with colloidal silicas. A gelcoat repair kit usually includes a gelcoat base and a catalyst.

Thoroughly blend the two components to create a consistent mixture. Using an applicator, apply the new gelcoat to the repaired area, ensuring a smooth and even application. It's recommended to slightly overfill the area to allow for sanding and buffing later.
Step 6: Cure and Finish

Allow the newly applied gelcoat to cure according to the manufacturer's recommended curing time. This typically involves waiting for the gelcoat to harden and reach its full strength. 

Once cured, use Norton Pro Plus A975 in 320 grit and progress to 600 grit to sand the repaired area, starting with a coarser grit and gradually moving to finer grits.

Bled the surface with Pure Ice Q175 in 800 and 1000 grit. 

Finally, polish and buff the surface using Farécla Profile 200 or 350 to achieve a glossy, seamless finish that matches the surrounding gelcoat. If the gelcoat is coloured, Farécla Profile 400 will better suit the required finish.


How to Repair Cracked and Crazing Gelcoat

Step 1: Clean and Assess the Damage

Clean the cracked area thoroughly to remove dirt, debris, or loose gelcoat fragments. Use a mild detergent or gelcoat-specific cleaner along with water. Rinse the area and allow it to dry completely.

Assess the cracks to determine their length, depth, and overall extent of damage.
Step 2: V-shaped Groove

Using a rotary tool and a mounted point, carefully create a V-shaped groove along the length of the crack.

Aim to go through the gelcoat but do not damage the laminate layer. The purpose of this groove is to provide a better bonding surface for the repair material and prevent the crack from reappearing in the future. Ensure that the groove is wide enough to accommodate the repair material.

Use MeshPower M920 sanding discs or MeshPower M920 sheets with a sanding block to smooth the surface further – start with 120 grit and progress to 240 grit to ensure the surface is as smooth as possible.

Step 3: Fill the Groove with Gelcoat

Prepare a gelcoat mixture with colloidal sillicas according to the manufacturer's instructions.

Apply the gelcoat mixture into the V-shaped groove, completely filling the crack. Use a putty knife or a plastic applicator to smooth the epoxy surface and remove excess material.

Allow the epoxy to cure as per the manufacturer's recommendations.

Step 4: Sand and Blend

Once the epoxy has fully cured, use Norton Pro Plus A975 in 320 grit to carefully sand down the repaired area.

Start with 320 grit and gradually move to 800 grit, ensuring a smooth transition between the repaired section and the surrounding gelcoat. Take care not to sand too aggressively, as this may damage the surrounding surface.

Blend the surface with Pure Ice Q175 in 1000 grit for the perfect finish before polishing.

Step 5: Polishing and Finishing Touches

Clean the surface and then allow the surface to dry thoroughly. Finally, polish and buff the surface using Farécla Profile 200 or 350 to achieve a glossy, seamless finish that matches the surrounding gelcoat.


Further Support

To summarise, repairing gelcoat blisters, cracks, and crazing is possible with the right knowledge and tools.

By following the instructions in this article, you can restore your boat's gelcoat and ensure its long-term protection.

Remember to perform regular maintenance and take preventive measures to avoid future damage, keeping your gelcoat in excellent condition.

For further information, download our dedicated Norton Marine Catalogue.

Watch our Norton Livestream on how to sand and finish a composite panel.


SGA image


If you need further assistance, use our Distributor Locator to find a local Norton product supplier or contact Norton directly for support.

Contact Us