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    Repairing Gelcoat Stress Cracks

    Repairing Gelcoat Stress Cracks

    Understanding how or why your boat's gelcoat cracks appeared in the first place is crucial for a successful repair. For instance, if the cracks were caused by hitting a seawall or dropping a champagne bottle on deck, fixing them will teach you how to avoid such incidents in the future: steer clear of activities that might lead to hitting the seawall and dropping bottles.

    However, some cracks seem to appear out of nowhere, often found in cabin and cockpit corners, around screw holes, or even in the middle of the deck. These cracks can be puzzling to diagnose.

    What is Gelcoat?

    Gelcoat is a pigmented, high-quality polyester resin used as the finishing layer on fiberglass boats. Like any material, it has its pros and cons. Let's explore some of the drawbacks that lead to cracking.

    After application, gelcoat shrinks by 4 to 7 percent during curing and can have less than 1 percent elongation. This shrinkage contributes to the hard, attractive finish we appreciate, but it also makes gelcoat brittle, especially if applied thicker than recommended.

    Gelcoat should be about 20 mils (.020 of an inch) thick when properly applied to the mold by the manufacturer. However, when building a boat deck with cabin top corners and seat backs, edges often receive more gelcoat than necessary. Gelcoat can also run and accumulate in inside corners, or it might be unevenly applied across the mold, leading to potential issues.


    Thick areas of gelcoat lack the reinforcing fabric needed to keep it together, leading to stress cracks. These can occur when the part is removed from the mold or even years later as the part experiences additional stress from flexing. Typically, these cracks show up as single-line gelcoat cracks in the cabin or cockpit. While these cracks are hard to avoid and part of boat ownership, the repair process is straightforward and simple, as I’ll detail later.

    Another common type of crack is the screw hole crack, which appears as a hairline crack or two around deck hardware. This often happens when a self-tapping or sheet metal screw is used to install hardware, even if a proper pilot hole was drilled. The pressure from the screw threads can cause the gelcoat to crack. Not using a pilot hole or using one that's too small exacerbates the problem. These cracks are frequently seen around snaps for canvas and beverage holders. To prevent this, drill a pilot hole and then countersink it to remove the gelcoat to the diameter of the screw threads. This reduces pressure on the gelcoat and creates a small pocket for sealant to secure the screw.


    Repairing Minor Gelcoat Cracks

    To repair minor gelcoat cracks, start by using a rotary tool or scraping tool to widen the crack into a “V” shape. You need to reach the fiberglass layer and open the entire length of the crack. Once this is done, use 80-grit sandpaper to roughen up the opening you’ve created. Folding the sandpaper will help you get it into the crack more effectively.

    After sanding, remove any dust from the area. Then, fill the crack with G/flex® 655, Six10®, or WEST SYSTEM 105 Epoxy Resin®/20X Hardener thickened with 406 Colloidal Silica Adhesive Filler. Once the epoxy has fully cured, wash it with water to remove any amine blush. This blush is water-soluble, so water is the best way to clean it off. After washing, sand the area and apply a color-matching gelcoat.

    If the gelcoat cracks were caused by the fiberglass flexing, it’s a good idea to add some fiberglass reinforcement to the backside to prevent future cracks.

    For screw hole repairs where there is a fiberglass core behind it, seal the core with epoxy. If the core is made of balsa or foam, use a bent nail to remove a small amount of the core from behind the fiberglass, and then fill the area with 105/20X thickened with 404 High-Density Adhesive Filler. For plywood or other wood cores, create an oversized hole and fill it with 105/20X and 404 High-Density Filler. Both methods provide a strong seal and more holding strength for fasteners. For more detailed instructions on fastener bonding, refer to the Fiberglass Boat Repair & Maintenance manual’s section on hardware bonding or visit Westsystems on our website. 

    Repairing Severe Gelcoat Cracks

    When you notice a concentrated group of cracks running in the same direction, it indicates a more severe problem. This often happens on the leading edge of the cabin top where it meets the deck. Such cracks can result from high flex areas enduring heavy seas or too much load on the boat. Cracks may also form at bulkheads or other hard spots behind the fiberglass when the boat flexes, creating pressure points.

    To repair this type of cracking, start by removing the gelcoat from the area and inspecting the fiberglass for deeper cracks. Wipe the area with a wet cloth; this will make cracks or fractured fiberglass appear as white lines in the laminate.

    Continue removing the damaged laminate until you reach undamaged fiberglass. Then, follow these steps for a traditional fiberglass repair:

    1. Grind Out the Area: Taper the area to a 12-to-1 ratio from the deepest point of the damage.
    2. Cut Fiberglass Patches: Start with the largest patch and work your way down to smaller ones to fill the ground-away area.
    3. Apply Epoxy and Fiberglass: Using WEST SYSTEM Epoxy, wet out the fiberglass pieces and apply them to the repair area, starting with the largest patch first and working your way to the smallest. Allow the epoxy to cure.
    4. Clean and Sand: Wash the area with water and a nylon scrub pad to remove any amine blush. Sand the repair to make it level with the surrounding area. If there are low spots, fill them using WEST SYSTEM Epoxy thickened with 407 Low-Density Fairing Filler as a fairing compound. Allow the epoxy to cure, wash it, and sand it to shape.
    5. Seal the Repair: After final fairing, apply two coats of unthickened WEST SYSTEM Epoxy to seal the repaired area.

    These steps will ensure a durable and long-lasting repair to your boat's gelcoat.

    Thermal Fatigue Gelcoat Cracks

    The environment can significantly affect gelcoat. While wax can protect it from fading, there's no way to shield gelcoat from the repetitive expansion and contraction caused by temperature changes. This movement can lead to cracking, which may appear in either a parallel or random pattern.

    Parallel patterned cracks vary in length from short to several inches and are spaced a few to several inches apart. These are often referred to as "old age cracks" and are typically caused by the expansion of the deck laminate, making the gelcoat more susceptible to flexural stress.

    Short, random cracks are known as gelcoat crazing. This can be localized to a small area or cover an entire deck. Cockpit soles sometimes resemble shattered car windows due to crazing, which is caused by the gelcoat expanding and contracting over a given area.

    We often receive customer inquiries about whether sanding the gelcoat and applying epoxy will fill the cracks and prevent them from returning. Unfortunately, the answer is no. Cracked gelcoat needs to be removed. After removing the gelcoat, sand the fiberglass laminate with 80-grit sandpaper. Once sanded, make any necessary repairs to the area. Then, apply a minimum of three coats of WEST SYSTEM® Epoxy to seal the fiberglass before applying a finish coat of paint or gelcoat.

    When gelcoat cracking is a symptom of an underlying problem, simply repairing the crack without addressing the root cause can lead to repeated cracking. For instance, if the gelcoat is too thick, thinning it is not a practical solution.

    Once repairs are completed correctly, some boats can remain crack-free for years. Taking your time and doing the job right will save you time and money in the long run.

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