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How To Fix Blisters In Gelcoat: Tips And Tricks

As a boater, you understand the importance of keeping your vessel in top condition. One issue that can arise is blisters in the gelcoat.

These small bumps can be unsightly and, if left untreated, can lead to more significant problems.

Fortunately, with the right knowledge and tools, fixing gelcoat blisters is a manageable task.

A gloved hand sanding a blistered area in gelcoat on a boat hull, followed by applying gelcoat filler and smoothing with a putty knife

Key Takeaways

  • Understanding the cause of gelcoat blisters is crucial to fixing them.
  • Assessing the damage is necessary to determine the scope of the repair.
  • Proper preparation and repair techniques can restore the appearance and integrity of your boat.

Understanding Gelcoat Blisters

A boat hull with visible gelcoat blisters, some open and oozing, others closed. Tools and materials for repairing blisters are nearby

Gelcoat blisters are a common problem for boat owners. They are caused by the build-up of water between the gelcoat and the fiberglass hull. If left untreated, they can lead to serious damage to the boat’s structure.

In this section, I will discuss the different causes of gelcoat blisters and some preventive measures that can be taken.

Causes of Gelcoat Blisters

There are several reasons why gelcoat blisters occur. One of the most common causes is osmotic pressure. This is when water is absorbed by the gelcoat and creates pressure that causes blisters to form. Other causes include:

  • Poor quality gelcoat
  • Inadequate preparation of the hull before applying the gelcoat
  • Exposure to extreme temperatures
  • Chemical exposure
  • Poor ventilation

Preventive Measures

Prevention is key when it comes to gelcoat blisters. Some measures that can be taken to prevent them from forming include:

  • Using high-quality gelcoat
  • Properly prepare the hull before applying the gelcoat
  • Using a barrier coat to prevent water absorption
  • Maintaining proper ventilation in the boat
  • Avoiding exposure to extreme temperatures and chemicals

In conclusion, understanding the causes of gelcoat blisters and taking preventive measures can help you avoid this common problem.

Gelcoat Blister Repair Process

1. Assessing the Damage

A boat with peeling gelcoat, revealing underlying damage

A/Inspection Techniques

Before repairing blisters in gelcoat, it is important to thoroughly inspect the affected area. you should start by cleaning the surface with soap and water to remove any dirt or debris.

Next, use a sharp tool like a scraper or a pick to carefully remove any loose or damaged gelcoat around the blister.

This will help me determine the extent of the damage and whether or not the underlying fiberglass is affected.

B/Determining Repair Scope

Once you have assessed the damage, you can determine the scope of the repair needed.

If the blister is small and only affects the gelcoat, you can simply sand it down and apply a new layer of gelcoat.

However, if the blister has penetrated the fiberglass, a more extensive repair will be required.

In some cases, it may be necessary to cut out the damaged area and replace it with new fiberglass. This is a more involved repair that requires more time and skill. It is important to remember that if the underlying fiberglass is compromised, it can weaken the overall structure of the boat.

Overall, a thorough inspection of the damaged area is crucial in determining the appropriate repair method. Taking the time to assess the damage will help ensure a successful repair and prevent further damage to the boat.

2. Preparation for Repair

A boat with gelcoat blisters sits on a stand. A person sands the affected area, applies epoxy, and smooths the surface in preparation for repair

A/Gathering Materials

Before starting the repair process, it is important to gather all the necessary materials. This will ensure a smooth and efficient repair process. Here are the materials you will need:

  • Gelcoat repair kit
  • Sandpaper (80, 120, 220 grit)
  • Acetone
  • Clean rags
  • Masking tape
  • Mixing cups and sticks
  • Paintbrushes

It is important to note that not all gelcoat repair kits are created equal. Make sure to choose a kit that matches the color of your boat’s gelcoat. This will ensure a seamless repair.

B/Surface Preparation

Proper surface preparation is crucial for a successful gelcoat repair. Here are the steps to follow:

  1. Clean the damaged area with acetone and a clean rag. This will remove any dirt, wax, or oil that may be present.
  2. Sand the damaged area with 80-grit sandpaper. This will roughen up the surface and provide a better bond for the gelcoat.
  3. Sand the area again with 120-grit sandpaper. This will smooth out any rough spots left by the 80-grit sandpaper.
  4. Sand the area one more time with 220-grit sandpaper. This will create a smooth surface for the gelcoat to adhere to.
  5. Mask off the area around the damaged area with masking tape. This will prevent any accidental damage to the surrounding gelcoat.

By following these steps, you will have properly prepared the damaged area for the gelcoat repair. With the necessary materials gathered and the surface properly prepared, you are now ready to move on to the next step in the repair process.

3. Repairing Gelcoat Blisters

A person sanding and filling gelcoat blisters on a boat hull

A/Drying the Affected Area

The first step in repairing gelcoat blisters is to dry the affected area. This is important because if the area is not completely dry, the repair will not adhere properly.

To do this, I use a heat gun or a hair dryer to dry the area thoroughly. Once the area is dry, Use a scraper or sandpaper to remove any loose or damaged gelcoat.

B/Applying Filler

After the area is dry and cleaned, the next step is to apply filler to the affected area. Use a two-part polyester filler, which is mixed according to the manufacturer’s instructions.

Apply the filler to the affected area with a putty knife, making sure to fill any voids or holes. Once the filler is applied, let it cure according to the manufacturer’s instructions.

C/Sanding the Repaired Area

Once the filler is cured, sand the repaired area with sandpaper. Start with coarse grit sandpaper and gradually move to finer grit sandpaper until the repaired area is smooth and level with the surrounding gelcoat.

After sanding, wipe the area with a clean cloth to remove any dust or debris.

So, repairing gelcoat blisters is a straightforward process that requires drying the affected area, applying filler, and sanding the repaired area.

==>> Also read my guide on removing rust stains from fiberglass boats here!

4. Applying New Gelcoat

A boat's hull being coated with new gelcoat to repair blisters

A/Mixing Gelcoat

Before applying a new gelcoat, it is important to properly mix it. I always start by reading the manufacturer’s instructions carefully. Then, I gather all the necessary tools and materials, including gelcoat, hardener, and a mixing cup.

So, you should do the same.

To mix the gelcoat, I pour the desired amount into the mixing cup. I then add the recommended amount of hardener and mix thoroughly.

It is important to mix the gel coat and hardener together quickly, as the mixture will start to harden within a few minutes.

B/Application Process

Once the gelcoat is mixed, it is time to apply it. Start by cleaning the area to be repaired with a solvent to remove any dirt or debris. Then, use a brush or a roller to apply the gelcoat in thin, even layers.

It is important to allow each layer to dry completely before applying the next one. I usually wait about 24 hours between layers.

After the final layer is applied, I wait for the gelcoat to cure completely, which can take up to a week depending on the temperature and humidity.

Overall, applying a new gelcoat is a straightforward process that requires attention to detail and patience.

5. Finishing Touches

A hand applies gelcoat over a surface, smoothing out blisters with a brush. Sandpaper and polish sit nearby for finishing touches


After the gelcoat has fully cured, it’s time to polish the surface to restore its shine. I recommend using a rotary buffer with a foam pad and a polishing compound.

Start with a low speed and gradually increase it until you get the desired results. Be sure to keep the buffer moving to avoid burning the gelcoat.

It’s important to work in small sections and use a clean microfiber towel to remove any excess compound.

Repeat the process until the entire surface is polished to your satisfaction. Avoid using harsh abrasives or compounds that could damage the gelcoat.


Once the surface is polished, it’s time to apply a protective wax coating.

I recommend using a high-quality marine wax that is specifically designed for gelcoat surfaces. Apply the wax in a thin, even layer using a foam applicator pad.

Allow the wax to dry to a haze, then buff it off using a clean microfiber towel.

Repeat the process until the entire surface is waxed. This will help protect the gelcoat from UV damage and other environmental factors.

Remember to always follow the manufacturer’s instructions when using any polishing or waxing products. With proper care and maintenance, your gelcoat surface will look great for years to come.

Long-Term Maintenance

1. Regular Inspections

As a boat owner, it’s important to conduct regular inspections of your gelcoat to identify any blisters early on. I recommend inspecting your gelcoat every six months or so, especially if you keep your boat in the water year-round.

During your inspection, look for any signs of blistering, such as raised bumps or small craters on the surface of the gelcoat.

If you do notice any blisters, mark their location with a piece of tape or a marker so you can keep track of them.

2. Ongoing Care

To prevent blisters from forming in the first place, it’s important to take good care of your gelcoat. Here are a few tips for ongoing care:

  • Wash your boat regularly with a mild soap and water to remove any dirt or grime that could lead to blistering.
  • Avoid using harsh chemicals or abrasive cleaners on your gelcoat, as these can damage the surface and make it more susceptible to blisters.
  • Apply a high-quality wax or polish to your gelcoat at least once a year to help protect it from the elements.
  • If you notice any small cracks or chips in your gelcoat, repair them as soon as possible to prevent water from seeping in and causing blisters.

By following these simple tips for long-term maintenance, you can help prevent blisters from forming on your gelcoat and keep your boat looking great for years to come.

Frequently Asked Questions

What steps are involved in repairing gelcoat blisters on a boat?

To repair gelcoat blisters on a boat, the first step is to remove the damaged gelcoat. This can be done by sanding or grinding the area until the affected layers are removed. Next, the area should be thoroughly cleaned and dried. Once the surface is prepared, a new layer of gelcoat can be applied to the affected area. It is important to follow the manufacturer’s instructions when applying the new gelcoat and to allow it to cure completely before using the boat again.

Can hull blisters lead to more serious damage if left untreated?

Yes, if hull blisters are left untreated, they can lead to more serious damage. Over time, water can penetrate the hull and cause delamination, which can weaken the structure of the boat. This can lead to costly repairs or even the need to replace the hull entirely. It is important to address any blisters as soon as they are noticed to prevent further damage.

What is the best method to repair osmosis blisters on fiberglass boats?

The best method to repair osmosis blisters on fiberglass boats is to first remove the affected gelcoat and any damaged layers of fiberglass. The area should then be allowed to dry completely before applying a layer of epoxy resin. Once the resin has cured, a new layer of fiberglass cloth can be applied and then covered with a layer of gelcoat. It is important to follow the manufacturer’s instructions when working with epoxy and fiberglass to ensure proper curing and adhesion.

What materials are recommended for filling and repairing blisters on boat hulls?

There are a variety of materials that can be used to fill and repair blisters on boat hulls, including epoxy resin, fiberglass cloth, and gelcoat. It is important to use materials that are compatible with the existing hull material and to follow the manufacturer’s instructions when working with these materials.

Is it necessary to repair gelcoat blisters on boats used in freshwater?

Yes, it is necessary to repair gelcoat blisters on boats used in freshwater. While freshwater does not contain the same level of salt and other minerals as saltwater, it can still cause damage to the hull if blisters are left untreated. In addition, blisters can be a sign of other underlying issues with the hull that should be addressed.

How can one prevent the formation of gelcoat blisters in the future?

To prevent the formation of gelcoat blisters in the future, it is important to properly maintain the hull of the boat. This includes regular cleaning and waxing, as well as addressing any damage or issues as soon as they are noticed. In addition, using high-quality materials and following manufacturer recommendations when applying gelcoat can help prevent the formation of blisters.

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