How to Repair a Fiberglass Hull on Boats

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How to Repair a Fiberglass Hull on Boats

For many boat owners, one of the worst things that can happen is damage to the hull. Hull damage can occur quite frequently either by running aground, coming into contact with objects at sea or due to rot. While a damaged fiberglass hull can sink any boat owners mood, it should be noted that the option of repairing a fiberglass hull is always available. Fiberglass does crack and can be damaged quite easily, however with a little time, energy and skill; you can have your fiberglass boat hull looking new again. Here are some tips.

For boat hulls that have experienced significant damage, you should haul your boat out of the water and do an intensive inspection on the entire hull. Extensive hull damage can cause your boat to ultimately flood and even possibly sink. For damage that is slight to moderate and above the water line, you can usually fix the hull while your boat is docked.

A good method to ensure that your hull is only slightly damaged is to tap the damaged area with a screwdriver. When you tap the damaged area, you should be looking to hear a solid noise. A solid noise usually means that the damage is not extensive; however, if the noise coming from the damaged fiberglass is hollow or dull, you are probably looking at deeper, more serious damage.

For fiberglass hulls with light damage, repairing the fiberglass usually takes only a few hours and for the most part can be done from inside the hull.

What you will need:

  • Screw driver
  • Pliers
  • Sharp scissors
  • Sand paper (36 grit)
  • Disk Sander
  • Dewaxing solvent
  • Cloth
  • Fiberglass repair kit (includes the fabric resin, gel coat and catalyst)
  • Eye protection
  • Lung protection (dusk mask or respirator)
  • Polyvinyl Alcohol
  • Polish
  • Paste wax
  • Formica
  • Resin roller

Step 1. Remove the Damaged Area

fiberglass repairFirst off inspect the damage and make either a circular or oval cut to remove the damaged area. Damaged fiberglass should not be reused, if an area is even slightly damaged remove it completely.

If your hull has a slight damage above the water level, you can fix the hole from the inside. For damage that is small (about 3 inches) you will want to refinish an area that is about 12 inches larger. So for 5 inches of damage, expect to finish an area about 17 inches in diameter. Repairing a hole from the inside is highly advantageous because you will do far less finishing work on the hull and you can lay up the repair with the gel coat first.

Step 2. Remove Wax

It is important to remove the wax before you start sanding and grinding the area. On the hull, you will usually find mold release and wax surfactant. The mold release is found on the outside of the hull, the wax surfactant is found on the interior of the hole. Use a dewaxing solvent to remove the wax.

Step 3. Sand and Grind

Sanding and grinding will ultimately help you get a stronger bond and repair. With a disk sander and sand paper grind the exterior area around the hole that was cut out including the bevel edge. In addition, grind the interior of the hole forming a rectangle that is slightly larger than the hole cut out.

It should be noted that grinding fiberglass can be dangerous due to the amount of small pieces of fabric that can become airborne. Fiberglass is extremely irritating to the skin; make sure you wear goggles, a good dust mask or respirator and clothing so that the fiberglass doesn’t come in contact with your skin. Once the area is grinded, make sure you remove any dust with a brush and cloth.

Step 4. Mask and Mold

It is a good idea to apply a heavy coat of paste wax before masking the hull. It should be noted that paste wax should only be used on the exterior of the hole and not on the edge of the hole or the interior. Using Formica cut a piece that is slightly larger than the hole. Before applying the Formica apply polyvinyl alcohol to it. Now apply the backer to the hull with tape. In most cases the Formica will easily assume the shape of the hull. If the mold does not take its form easily, use a heat gun to gently heat the Formica to its form.

Step 5.Cut the Fiberglass Fabric

Using a fiberglass repair kit, cut the fiberglass fabric to about 1 inch wider than the size of the hole. You will probably want to layer the fabric as follows: two layers of 1.5 oz. mat followed by 6 oz. cloth. Depending on the thickness of your hole, you can determine the amount of layers you will need.

Step 6. Apply Resin

Apply resin and allow the resin to cure. Resin cures with the help of a catalyst. Follow the instructions that come with each fiberglass repair kit. It is important to note that damage to hulls below the water line should use epoxy. Epoxy is much stronger than resin. It should also be noted that those who use epoxy cannot use gel coat.

Step 7. Apply Gel Coat

Gel coat is a colored polyester resin. Brush the gel coat onto the waxed backer. You should brush on the gel coat to make it about 20 mm thick. Besides brushing on a gel coat, another option is to spray it on.

Step 8. Lay Up

During the lay up, you will want to wet two layers of mat and one layer of cloth using a polyester resin. Once the resin is applied, compress it against the gel coat. Work the mat so that you remove all bubbles from the resin. You can accomplish this using a resin roller.

Apply the Finish

It is easy to apply a finish; cut out a rectangular piece of mat that is about one inch larger than the cloth applied. Smooth out this mat with a resin roller or squeegee. Now seal the top layer of the mat with PVA alcohol to allow it to cure.

Once the damaged area has cured, look for imperfections and fill with gel coat. Allow the gel coat to fully cure. Once cured, all that is left is to re-sand the area if necessary and make the repaired area nice and polished.

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  1. Dave said,

    on July 22nd, 2009 at 12:41 pm

    I have a 40′ carver aft cabin that hit a pieling in the starbord corner and sunk
    I’m repariing the boat. You said to use epoxie
    under the water line, but you cant gelcoat epoxie ? I need some typ of backer to shape the corner can you help me ???

    Dave 978-355-4424