How to Fix Spider Cracks in Fiberglass Boat

While spider cracks in fiberglass boats may seem minor, they can cause serious damage if left untreated. This blog post will show you how to fix spider cracks in fiberglass boats using a few simple steps. We will also provide tips on preventing them from happening in the first place. Read on to learn more.

How to Fix Spider Cracks in Fiberglass Boat

Summary: Fixing spider cracks in your fiberglass boat doesn’t need to be stressful. Start by cleaning the area and cutting out the cracked fiberglass. Then apply epoxy resin and a fiberglass repair kit. Allow it to dry before sanding the area. If you want, you can add two coats of resin, a gelcoat, primer and paint to finish off the job. Finally, wax the area and enjoy your boat!

What Causes Spider Cracks in Fiberglass Boats?

There are a few different things that can cause spider cracks in fiberglass boats. One of the most common causes is exposure to sunlight. The UV rays from the sun can cause fiberglass to weaken and crack over time. Another common cause is impact damage. If you hit something while out on the water, it can cause the fiberglass to crack. Finally, age can also be a factor. If your boat is getting up there in years, it is more likely to develop spider cracks.

Why Are Spider Cracks Dangerous?

Spider cracks may seem minor, but they can lead to serious damage if left untreated. The cracks can allow water to seep into the hull of your boat, which can lead to leaks and other issues. Additionally, the cracks can weaken the overall structure of your boat, making it more likely to tip over or sink. Therefore, if you have spider cracks in your boat, fixing them as soon as possible is important.

Things You’ll Need

  • Fiberglass repair kit
  • Epoxy resin
  • Mixing cup
  • Paint stirrer
  • Sandpaper
  • Safety goggles
  • Gloves

A Complete Guide on How to Fix Spider Cracks in Fiberglass Boat

Step 1: Assess the Damage and Determine Repair Method

Inspect the spider cracks on your fiberglass boat to assess the extent of the damage. Spider cracks are often cosmetic and can be repaired using a gelcoat repair kit. However, if the cracks penetrate the fiberglass layers or are accompanied by structural damage, a more extensive repair involving fiberglass reinforcement may be necessary.

Step 2: Gather Necessary Tools and Materials

To repair spider cracks in a fiberglass boat, you will need various tools and materials, such as:

  • Gelcoat repair kit (containing gelcoat, hardener, and pigments)
  • Fiberglass repair kit (containing fiberglass cloth, resin, and hardener), if needed
  • Sandpaper (80-grit, 220-grit, and 400-grit)
  • Orbital or palm sander (optional)
  • Mixing sticks and cups
  • Plastic spreaders or putty knives
  • Masking tape
  • Clean rags
  • Acetone or rubbing alcohol
  • Protective gloves and safety glasses
  • Dust mask and proper ventilation

Step 3: Clean and Sand the Damaged Area

Using a clean rag and acetone or rubbing alcohol, clean the area around the spider cracks to remove any dirt, wax, or debris. Then, sand the area using 80-grit sandpaper or an orbital sander to remove the damaged gelcoat and create a rough surface for the repair material to adhere to. Be sure to sand at least an inch beyond the edge of the cracks to ensure proper adhesion.

Step 4: Apply Masking Tape Around the Repair Area

Apply masking tape around the perimeter of the repair area to protect the surrounding gelcoat from accidental damage during the repair process.

Step 5: Mix the Gelcoat

Using the gelcoat repair kit, mix the gelcoat, hardener, and pigments according to the manufacturer’s instructions. Ensure that the color matches your boat’s existing gelcoat as closely as possible.

Step 6: Apply the Gelcoat to the Spider Cracks

Using a plastic spreader or putty knife, apply a thin layer of the mixed gelcoat to the spider cracks. Press the gelcoat firmly into the cracks to ensure proper adhesion and coverage. Be sure to apply the gelcoat evenly and smoothly to minimize the need for sanding later.

Step 7: Allow the Gelcoat to Cure

Allow the gelcoat to cure according to the manufacturer’s recommended time. This can vary depending on factors such as temperature, humidity, and the thickness of the applied gelcoat.

Step 8: Sand the Cured Gelcoat

Once the gelcoat has fully cured, sand the repair area using 220-grit sandpaper to remove any excess material and smooth the surface. Be careful not to sand through the new gelcoat and expose the fiberglass beneath.

Step 9: Apply Additional Gelcoat Layers if Needed

If the initial layer of gelcoat does not fully cover the spider cracks or provide an even surface, apply additional layers as needed, allowing each layer to cure before sanding and applying the next.

Step 10: Wet Sand the Repair Area

After the final layer of gelcoat has cured, wet sand the repair area using 400-grit sandpaper to achieve a smooth and glossy finish. Be sure to keep the sandpaper wet throughout this process to avoid scratching the gelcoat.

Step 11: Buff and Polish the Repair Area

Using a buffing compound and a polishing pad or soft cloth, buff the repair area to restore the shine and blend the repaired area with the surrounding gelcoat.

Step 12: Repair Structural Damage if Necessary

If the spider cracks are accompanied by structural damage or penetrate the fiberglass layers, follow the instructions provided in the fiberglass repair kit to reinforce the damaged area with fiberglass cloth and resin. This process will typically involve the following steps:

a. Cut the fiberglass cloth to the appropriate size and shape to cover the damaged area, allowing for an overlap of at least one inch on all sides.

b. Mix the resin and hardener according to the manufacturer’s instructions.

c. Apply a layer of the mixed resin to the sanded and cleaned repair area using a brush or plastic spreader.

d. Lay the fiberglass cloth over the resin, ensuring that it is properly aligned and smooth.

e. Apply another layer of resin over the fiberglass cloth, ensuring that it fully saturates the cloth and that there are no air bubbles trapped beneath.

f. Allow the resin to cure according to the manufacturer’s recommended time.

g. Once the resin has cured, sand the repaired area with 80-grit sandpaper to remove any excess material and create a smooth surface.

h. Proceed with applying gelcoat, as described in Steps 5-11, to finish the repair and match the surrounding surface.

Step 13: Inspect the Repair and Perform Regular Maintenance

After completing the repair, inspect the area to ensure that the spider cracks have been properly addressed and that the repaired area blends seamlessly with the surrounding gelcoat. Regularly inspect your boat for any new spider cracks or other damage and address any issues promptly to maintain the integrity and appearance of your fiberglass boat.

By following these steps and using the appropriate materials and techniques, you can effectively repair spider cracks in your fiberglass boat and restore its appearance and structural integrity. Regular maintenance and prompt attention to any damage will help extend the life of your boat and ensure that it remains in optimal condition for years to come.

You Can Check It Out To Fix Resin That Won’t Harden

How Much Does It Cost to Fix Spider Cracks in a Fiberglass Boat?

The cost of repairs will vary depending on the severity of the damage. For minor cracks, you can expect to pay around $50. For more severe damage, you could end up paying upwards of $1,000. If you have a lot of spider cracks, it is best to consult with a professional to get an estimate.

How to Prevent Spider Cracks in Fiberglass Boats?

The best way to prevent spider cracks is to perform regular maintenance on your boat. This includes washing and waxing the boat regularly. It would be best if you also inspected the hull for any cracks or damage that could allow water to seep in and cause the spider cracks. If you do find any damage, repair it immediately to prevent further cracking. Finally, store your boat in a dry, protected area during the off-season to prevent the hull from becoming damaged. Following these simple tips can help prevent spider cracks from forming in your fiberglass boat.

You Can Check It Out To Fix Uncured Epoxy Resin

Frequently Asked Questions

Can You Gelcoat Over Spider Cracks?

Unfortunately, no. Gelcoating can only be applied over completely solid surfaces. Spider cracks are not completely solid, and so Gelcoat would not adhere to them properly.

What Causes Spider Cracks in Fiberglass Boats?

Spider Cracks in Fiberglass Boats are most commonly caused by moisture infiltration and poor joint composition of the boat’s surface. Moisture infiltration can come from a number of sources, but one of the most common is rainwater seeping through cracks or gaps in the boat’s exterior skin. In turn, this increases water levels on the interior surfaces which causes them to swell and contract faster than they should.

This leads to stress fractures that form along these seams primarily because they’re under tension (from lifting).

The presence of organic material also plays a role in spider cracking since it makes joints more prone to swelling and irritation due to its absorptive qualities. As a result, storing your boat outside where it will be exposed to sunlight and air pollution is recommended for optimal protection against spider cracking.

Can a Spider Crack Be Repaired?

Yes, a spider crack can often be repaired with a little bit of silicone sealant. Silicone sealant is a type of glue that is used to fill in cracks, seams, and other openings in materials. It is easy to apply and dries quickly so that it can be filled and sanded in minutes.

Are Spider Cracks Normal in Boats?

Spider cracks are not normal in boats and should be fixed as soon as possible. spider cracks form when the fiberglass hull flexes or twists due to load, wave action, or an inland event such as a landslide. Over time, these small tears can gradually enlarge until they become large enough that water can leak into the boat through them. This creates corrosive conditions that can damage both the hull and onboard systems.

Spider cracks may also weaken the deck leading to further deterioration of your boat’s structural integrity over time. If you see any spider crack on your boat, don’t wait – get it repaired right away!


So, there you have it. Everything you need to know about how to fix spider cracks in fiberglass boat: remember to use the right products and tools for the job and take your time to make sure the repair is done properly. With a bit of patience and some basic skills, you can fix those pesky spider cracks and get back on the water in no time.

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