Amanda Knapp
Stress cracks in fiberglass boats are a common issue, often caused by the repetitive expansion and contraction of temperature changes, or by the gelcoat becoming brittle. These cracks can be repaired by grinding down the cracks to reach clean fiberglass, then filling the gap with glass or compound and re-gelcoating. It is important to identify the cause of the cracks to prevent them from reoccurring. For example, if the cracks were caused by fiberglass flexing, adding fiberglass reinforcement to the backside can help prevent future cracks. There are also commercial products available that are designed to fill cracks, such as MagicEzy Hairline Fix and Captain Tolley's Creeping Crack Cure.
How to fix stress cracks in a fiberglass boat
| Characteristics | Values |
|---|---|
| Cause of cracks | Could be due to repetitive expansion and contraction from temperature change, or from the gelcoat becoming brittle |
| Location of cracks | Most common on corners and radius areas where the layup of fiberglass is thinner and the resin amount is higher than it should be |
| Type of cracks | Stress cracks, spider cracks, hairline cracks, gelcoat crazing |
| Repair methods | Grind down cracks until "clean" fiberglass is reached, fill the gap with glass/compound and re-gelcoat; remove gelcoat and repair with fiberglass patches and epoxy; channel out cracks, clean with a solvent, and fill with gelcoat, polyester filler, or polyester resin and fiberglass strands |
| Prevention | Add fiberglass reinforcement to the backside, seal core with epoxy, and fill with high-density adhesive filler |
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What You'll Learn
Identify the cause of the cracks
It is important to identify the cause of the cracks in your fiberglass boat to ensure that the appropriate repairs are carried out. Stress cracks can be caused by a variety of factors, and understanding these causes will help you make informed decisions about the repair process.
Firstly, it is crucial to differentiate between a gel coat crack and a scratch. While scratches in the gel coat may be unsightly, they are merely surface blemishes. On the other hand, cracks penetrate through the gel coat and indicate some level of structural failure beneath. The main causes of gel coat cracks are stress and movement. The gel coat is hard and brittle, while the underlying glass fibre laminate is softer and more flexible. Therefore, when the laminate bends or moves, the gel coat, due to its brittleness, cannot bend with it and cracks.
The patterns of cracks can provide valuable clues about their underlying causes. For instance, star cracks, which spread out in a starburst pattern, are typically caused by external impact, with the central area being the point of impact. They can also be caused by flexing around a point, such as a stanchion base that has been leaned on or pulled. Linear cracks, which tend to run in parallel lines, indicate damage caused by bending or flexing. These cracks often occur along deck edges, where the deck meets the cabin sides, or around the edges of cockpit lockers.
Additionally, cracks in fiberglass boats can be caused by improper design or imposed stresses from within or outside the boat. Another factor is the intended cured hardness of the gel coat; harder gel coats are more durable but also more brittle and prone to cracking. Moreover, cracks may be caused by the builder's failure to counter-sink or pre-drill screw holes.
Identifying the cause of the cracks is essential to ensure effective and long-lasting repairs. By understanding the underlying causes, you can address any structural issues and prevent the cracks from reappearing. Therefore, it is recommended to thoroughly inspect the boat and identify the patterns and characteristics of the cracks to determine their root cause before proceeding with any repair work.
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Toughen the laminate
To toughen the laminate, you must first identify the cause of the cracks. For instance, cracks can be caused by a stanchion base that flexes, or they can spread out from a radius that vibrates or twists in a seaway. They can also be caused by the repetitive expansion and contraction of temperature change. If the cracks are caused by the fiberglass flexing, you can add some fiberglass reinforcement to the backside to help prevent future cracks.
The next step is to solidify the laminate by thickening it with more fiberglass, replacing coring materials, or adding structure. This is important because if you don't, new cracks will most likely appear, and the problem will persist. When repairing a cored laminate, it is critical that the scarf angle is set at a minimum ratio of 12:1 and that the patch thickness matches the original laminate. This will ensure that the stiffness will be the same as the surrounding area, reducing the chance of a crack around the repair area.
If the cracks are caused by a screw hole repair, you can 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, then fill the area with 105/20X thickened with 404 High-Density Adhesive Filler. For plywood or other wood cores, make an oversized hole and fill it with 105/20X and 404 High-Density filler. These methods will provide a good seal and more holding strength for the fasteners.
Finally, when repairing the laminate, it is important to use the right materials. Polyester-based resins work better with CSM than epoxy. Epoxy can be used on fabric as it is held together with stitching, not glue. For a protective layer, you're better off putting the glass on the outside.
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Clean the crack with a solvent
To clean the crack with a solvent, start by identifying the location and extent of the stress cracks. Stress cracks can occur on the surface of the hull or deck, particularly in corners and radius areas where the fiberglass layup is thinner and there is a higher resin concentration. These cracks can be long parallel lines or short random lines, and they may or may not penetrate the underlying fiberglass.
Once the cracks are located, use a wet cloth to wipe the area. This will help to reveal the full extent of the cracks, as the water will seep into the cracks, making them appear as white lines in the laminate. Continue wiping until all the fractured laminate is visible. If the cracks are merely cosmetic and do not penetrate the gelcoat, simply wipe the area with a solvent like acetone to clean the crack and prepare it for the next steps.
If the cracks are deeper and threaten the underlying laminate, further action may be required before cleaning with a solvent. In some cases, it may be necessary to grind out the cracks with a die grinder or Dremel tool to create a V-shaped groove that extends down to the fiberglass. This will help to ensure that the repair material adheres properly and does not recrack. However, it is important to be careful when grinding into the cracks to avoid damaging the fiberglass or affecting the flexural stress and strain spread.
After wiping the area with a wet cloth and ensuring that all fractured laminate is visible, clean the crack with a solvent. Use a solvent such as acetone, wiping it into the crack with a clean cloth or brush to remove any dirt, grease, or debris. This step is crucial for preparing the surface for the next steps, as it ensures that the repair materials will properly adhere to the crack and create a strong bond.
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Fill the crack with gelcoat or resin
Stress cracks in fibreglass boats are common and are often caused by the gelcoat becoming brittle, either at the factory or due to exposure to temperature changes. To fix these cracks, you can fill them with gelcoat or resin.
First, you need to widen the cracks into a "V" shape using a rotary tool, a scraping tool, or a Dremel or orbital sander. You will need to go through the gelcoat to the fibreglass and open up the full length of the crack. Then, use 80-grit sandpaper to scuff up the inside of the crack. Remove the dust with a solvent like acetone, and then you can fill the crack with gelcoat or resin.
If the cracks are shallow, you can fill them with gelcoat. Mix up a couple of small batches of gelcoat to help you match the colour of the surrounding area. Then, use a plastic putty knife to fill the crack. Cover the repair with plastic film as gelcoat does not cure when exposed to air. Once cured, you can sand and apply a colour-matching gelcoat to the area. If the cracks were caused by fibreglass flexing, add some fibreglass reinforcement to the backside to prevent future cracks.
If the cracks are deeper, you can fill them with resin. Wet out fibreglass cloth strips or mesh with resin and pack them into the crack, working out air pockets from the centre to the edges. Allow the resin to partially cure, then fill the remaining crack and blend the edges with thickened resin, overfilling slightly. Once cured, sand the area so it is even with the surrounding surface.
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Sand and finish the repair
Once you have identified the cracks and the underlying cause, and repaired the cracks using filler, resin, or epoxy, you can begin to sand and finish the repair.
Firstly, if you have used epoxy to repair the cracks, you will need to wash and sand the cured epoxy seal coat. You can then apply a topcoat of paint or gelcoat. If you have used gelcoat to repair the cracks, you will need to temporarily cover the repair with plastic film, as gelcoat does not cure thoroughly when exposed to air.
After the filler, resin, or epoxy has cured, you can begin to sand the repair. Start with dry sanding, using a smooth grade of paper, such as 220-grit. Then, move on to wet sanding, using increasingly refined grades of paper. This will help you achieve a smooth, even finish. Be careful not to over-sand the area, as this can damage the repair and the surrounding surface.
Finally, you can apply a finish to the repair. If the crack was caused by fiberglass flexing, you may want to add some fiberglass reinforcement to the backside of the repair to prevent future cracks. You can also use a topcoat of paint or gelcoat to finish the repair and protect it from the environment. Wax can be used to protect the gelcoat from fading, but it will not protect against the repetitive expansion and contraction caused by temperature changes, which can lead to cracking.
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Frequently asked questions
Stress cracks are usually found on the leading edge of the cabin top where it meets the deck, or on the hull or deck. They are often a result of a high-flex area enduring heavy seas or too much load on the boat. They can also be caused by the repetitive expansion and contraction of temperature change.
First, identify the cause of the cracks. Then, toughen the laminate by solidifying it with more fiberglass, replacing coring materials, or adding structure. Grind out the cracks and wipe them clean with a solvent like acetone. Then, fill the cracks with gelcoat, polyester filler, or polyester resin and fiberglass strands, depending on the depth of the cracks. Finally, sand and finish the repaired area.
You can use products such as MagicEzy Hairline Fix and Captain Tolley's Creeping Crack Cure to fix hairline cracks. However, these may not be effective substitutes for a more thorough repair job. You can also use flexible epoxy to fill the cracks after grinding them out. For a more permanent solution, you can use materials such as laminating polyester resin, fiberglass chopped strand matting, and AdTech P-14 Ultra Filler.
