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How to Repair a Fiberglass Boat Hull From the Outside? – 6 Steps

Written by Anthony Roberts / Fact checked by Jonathan Larson

how to repair a fiberglass boat hull from the outside

Knowing how to repair a fiberglass boat hull from the outside is a crucial skill all boat owners must master. From emergency repairs on the high seas to annual maintenance checks on dry land, competence in fixing leaky issues is desirable for operators of vessels with fiberglass hulls.

Never fret if you’re clueless about how to fix fiberglass boat units because we’ll outline the steps to keep your watercraft seaworthy. Let’s begin.

Step-by-step to Repair a Fiberglass Boat Hull From the Outside


Preparation steps:

1. Tools and materials

Here’s a list of resources to repair small hole issues in your fiberglass boat. Please note you might need other materials, depending on the problem.


  1. Fiberglass mats
  2. Epoxy or polyester resin
  3. Acetone
  4. Gelcoat
  5. Orbital sander with disc
  6. Sandpaper
  7. Rubbing compound
  8. Fiberglass mat cutting tool
  9. Paintbrush
  10. Mixing bowl or cup
  11. Marker
  12. Rugs
  13. Plastic wrap
  14. Tape
  15. Cardboard

2. Assessing the damage


Repairing fiberglass hulls starts with a comprehensive damage assessment. Patching a small hole or crack on the boat’s outside surface is easy and solves the issue instantly.

However, a sizable opening in the hull, especially beneath the waterline, requires more than an emergency patch. These issues need significant work on both sides of the hull to ensure structural integrity.

The steps we’ll outline in the following section address hull issues you can manage yourself. We recommend asking a professional or a boat mechanic for help if the damage is too severe.

Steps to fix a fiberglass boat hull from the outside

1. Emergency Repairs


You’re enjoying a pleasant cruise in the sea until you notice a small leak gushing into the boat’s interior. Your job is to mitigate the issue, allowing you to maneuver the watercraft to the safety of a marina. Here’s how.

  • Get a “collision mat” and place it over the hole. A collision mat is a marine-grade heavy-duty tarpaulin you can pull under the vessel and “patch” the hole on the hull from the outside.
  • Secure the collision mat lines to stable and sturdy boat structures.
  • Alternatively, you can apply a Flex Seal (a band-aid-like waterproof tape) over the hole if you don’t have a collision mat. It’s one of the handiest small fiberglass repairs boat owners can have.

 2. Long-Term Repairs


More substantial hull damage requires a more permanent solution. Hence, boat owners must prepare to repair large hole issues in their fiberglass watercraft.

You have two options: fix the problem with the boat in the water or take the vessel out of the water and repair the issue on dry land. Here are the steps.

  • Assess the leaky section and determine its position relative to the waterline.
  • Pick an in-water repair method if the damage is minor and above the waterline.
  • Choose an out-of-water repair for substantial structural issues below the waterline.

3. In-water Repairs


The principal advantages of in-water fiberglass boat hull repairs are convenience and practicality. Boat owners don’t need to take their watercraft out of the water and lift it onto a platform.

Unfortunately, this hull repair type is only suitable for minor, above-the-waterline issues. Here’s how to fix such problems.

  • Get a high-quality marine-grade caulk and read the application instructions.
  • Clean the hull surface to remove contaminants.
  • Apply the caulk to the crack or small holes, ensuring adequate filling.
  • Allow the caulk to cure before using the boat. We recommend checking the manufacturer’s curing time guidelines.

4. Out-of-water Repairs

We don’t recommend a DIY fiberglass boat repair for extensive structural damage. However, you can save hundreds of dollars by observing the following steps if your boat’s issue is a small- to medium-sized hole or crack in the fiberglass hull below the waterline.

  • Step 1. Prepare the hole.


Grab an orbital sander with a sanding disc and start smoothening the hole’s edges to reveal the underlying fiberglass. Ensure to create a tapering hole, with the inside edges narrower by three inches than the outermost edges.

Clean the hole and adjacent surfaces to remove dust and particulates. Soak a piece of cloth with acetone and rub it against the hole and the surroundings. Discard the acetone-drenched rug.

Cut the cardboard following the hole’s shape, ensuring you have a two-inch allowance from its edges. Get the plastic wrap and cover the cardboard cutout with it. Secure the plastic wrap with tape.

Insert the plastic wrap-covered cardboard into the hole, ensuring it adheres to and conforms to the opening’s shape. Secure it to the hull’s inside surface with masking tape.

  • Step 2. Prepare fiberglass patches and polyester resin.


A single fiberglass cloth layer is thin. You’ll need to create a meshwork of fiberglass patches to produce a strong framework for the damaged hull.

Cut several pieces of fiberglass cloth, preferably 10-ounce roving, with increasingly larger sizes. The bottommost layer should be flush with the opening’s inside diameter. Succeeding layers are bigger than the ones before them.

The topmost patch should be fiberglass cloth, ensuring it blends with the rest of the fiberglass hull.

Get your polyester resin and read the instructions for mixing. This fiberglass repair kit includes the polymer and a hardener, requiring you to combine predetermined proportions.

Pro Tip: Make small batches of the resin because this material hardens quickly. You can retard the hardening by working in an above-60-degree environment.

  • Step 3. Apply the fiberglass patches over the hole.


Lay the smallest fiberglass patch over the plastic-covered cardboard. Dip the paintbrush in the prepared polyester resin and apply over the first patch, saturating the fiberglass.

Place the second smallest fiberglass patch over the first and cover it with the resin. Press the fiberglass cloth along the sides, ensuring it sticks to the hull.

Repeat the process for the remaining fiberglass patches, ensuring to massage the edges of each layer to the hole’s sides. You will want the last roving piece to be flush with the fiberglass hull.

Lastly, cover the fiberglass patch layers with the fiberglass cloth. Use the paintbrush to spread the excess resin (from the underlying patches) over this layer.

Allow the fiberglass mesh to dry for ten hours.

Pro Tip: You might want to read the manufacturer’s curing time recommendations.

  • Step 4. Clean the fiberglass patch.


The next step is a repeat of Step 1, although the objective is to flush the fiberglass patch with the rest of the hull. You want to maintain your vessel’s hydrodynamic profile to ensure efficient movement in the water.

Sand the surface to make it even with the hull. Wipe off sanding debris and finish it off with acetone. Please remember to clean the surrounding area of the patched hole.

  • Step 5. Apply the gel coat.


Get the gel coat product and read the manufacturer’s application instructions. This item is necessary to ensure an identical texture and color to the hull’s original state.

Apply the gel coat over the repaired hull and cover the layer with plastic wrap to ensure a flawless gel coat surface. Press on it and cautiously remove wrinkles and air bubbles.

You can add two more layers of plastic wrap, eliminating air pockets and crinkles with each addition. You should see an improvement in the hull’s before and after pictures after drying the gel coat for ten hours (at least).

  • Step 6. Clean and polish the fiberglass hull.


You’re ready for the final smoothening of the fiberglass hull. This action is necessary to remove imperfections on the gel coat.

Remove the plastic wrap layers and wet-sand the surface with ultra-fine sandpaper or a fine-disc orbital sander.

Soak a cloth in clean water and clean the repaired hull section. Apply a rubbing compound (preferably one specific for fiberglass) and polish the area with the orbital buffer.

Apply wax and buff the surface to protect it and make it shiny. You might also want to check this video from DIYeasycrafts to help you master fixing a fiberglass boat hull.

How to Find a Leak in a Fiberglass Boat


Knowing how to find a leak in a fiberglass boat is crucial. You can repair deep scratch issues, but cracks and holes are different because they let water inside the vessel.

Steps to detect leaks

Start your search in the boat’s most vulnerable locations, such as scupper hoses, thru-hulls, bottom sections, and the waterline’s immediate vicinity.

Pour baby powder on damage-suspected areas and look for signs of dampness.

Clean and scrub every square inch of the boat to reveal holes or cracks that might look like ordinary dirt.

Common types of hull damage

While searching for leaks in the fiberglass boat, you should increase your knowledge of the common types of hull damage.

  • Tear – Similar to cracks, this damage often results from collisions that break the laminate and fiberglass fibrils.
  • Puncture – We know these as “holes” and are often smaller than tears. This damage can affect all fiberglass layers or only the superficial ones.
  • Crushed core – Blunt forces can compress the laminate and fiberglass fibrils, causing indentations without noticeable surface damage.
  • Delamination – Inadequate surface protection and extreme UV exposure can peel the fiberglass layers, undermining the hull’s integrity.

Maintenance and Preventive Measures


Although safe boating practices are sufficient to protect fiberglass hulls, experienced boaters know that observing proven maintenance and preventive measures are equally crucial.

  • Observe an annual fiberglass boat hull assessment to identify tiny cracks and other potential problems and fix them before they become severe.
  • Wash and clean your fiberglass hull frequently to remove mold, mildew, and other surface contaminants.
  • Apply marine-grade wax to improve the fiberglass’s UV protection.
  • Replenish the gel coat to enhance protection against the sun.
  • Apply barrier coating at the fiberglass boat’s hull below the waterline to prevent delamination and other fiberglass issues.

Helpful Tips


How long does it take to repair a fiberglass boat hull?

Emergency repairs should take only a few minutes. Meanwhile, off-the-water fiberglass boat hull repair might take at least 21 hours.

Can I repair a boat hull without professional assistance?

Yes, you can repair a boat hull without professional assistance, provided the damage is minor (i.e., cracks and small holes). 

What are the costs associated with DIY boat hull repair?

Repairing a fiberglass boat hull yourself can entail materials-related costs. You’ll buy a fiberglass repair package with fiberglass, resin, hardener, and other tools. You might also purchase sandpaper, orbital sander, acetone, and other resources.

How often should I inspect my boat hull for damage?

A once-yearly inspection of the fiberglass boat hull is sufficient to assess its integrity and discover any signs of damage.


Learning how to repair a fiberglass boat hull from the outside is a skill all boat owners should have. Although emergency hull repairs provide temporary relief in the water, allowing boaters to navigate their watercraft to safety, mastering more permanent solutions is crucial.

Boaters can fix structural issues above the waterline without taking their vessel out of the water. However, holes, cracks, and other problems below the waterline require operators to work on their watercraft on dry land.

We hope the six-step process outlined in this article made you more confident in reclaiming your fiberglass boat hull’s integrity.

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