How to Master Gelcoat Blisters on Yachts and Motor Cruisers
If you own a yacht or a motor cruiser, you may have encountered the dreaded problem of gelcoat blisters. These are unsightly bumps that appear on the underwater hull of your boat, caused by water penetrating the gelcoat and reacting with the underlying fibreglass Gelcoat blisters can affect the performance, appearance and value of your boat, and if left untreated, they can lead to more serious damage such as delamination and osmosis. So how can you master gelcoat blisters and find a solution to overcome the problem? In this blog post, we will explain what causes gelcoat blisters, how to prevent them, and how to repair them.
What Causes Gelcoat Blisters?
Gelcoat is the shiny outer layer of your fibreglass boat that protects it from UV rays, abrasion and water ingress. However, gelcoat is not entirely waterproof, and over time, small amounts of water can seep through its slightly porous surface and reach the fibreglass substrate beneath. This water can react with uncured chemicals in the fibreglass resin, creating acidic by-products that form pockets of fluid under the gelcoat. These pockets exert pressure on the gelcoat, causing it to bulge and form blisters.
The rate and severity of blister formation depend on several factors, such as:
The quality and thickness of the gelcoat
The quality and curing of the fibreglass resin
The temperature and salinity of the water
The duration and frequency of immersion
The presence of microorganisms or contaminants in the water
Gelcoat blisters can vary in size, shape and depth, ranging from pinheads to dinner plates. They can appear anywhere on the underwater hull, but are more common near the waterline, where the temperature fluctuations are greater. Gelcoat blisters are more prevalent in warm climates, where the water is more conducive to chemical reactions.
How to Prevent Gelcoat Blisters?
The best way to prevent gelcoat blisters is to keep your boat dry as much as possible. This means storing your boat on land or on a trailer when not in use, or lifting it out of the water periodically for inspection and maintenance. You should also wash your boat regularly with fresh water to remove any salt or dirt that may accelerate blister formation.
Another way to prevent gelcoat blisters is to apply a barrier coat over the gelcoat. A barrier coat is a layer of epoxy resin that forms an almost impermeable seal over the gelcoat, preventing water from reaching the fibreglass. A barrier coat can be applied when your boat is new or after repairing existing blisters. However, before applying a barrier coat, you need to make sure that your hull is completely dry and free of any blisters or defects.
To apply a barrier coat, you need to follow these steps:
Sand the gelcoat lightly with 80-grit sandpaper to create a rough surface for better adhesion.
Wipe the hull with acetone or a similar solvent to remove any dust or grease.
Mix the epoxy resin and hardener according to the manufacturer’s instructions.
Apply the epoxy resin with a roller or a brush, covering the entire hull with a thin and even coat.
Wait for the epoxy resin to cure according to the manufacturer’s instructions.
Repeat steps 3 to 5 until you have applied at least four coats of epoxy resin.
Sand the final coat lightly with 120-grit sandpaper to smooth out any bumps or runs.
Apply an antifouling paint over the epoxy resin to protect it from marine growth.
How to Repair Gelcoat Blisters?
If you already have gelcoat blisters on your boat, you need to repair them as soon as possible to prevent further damage. The repair process involves grinding out the blisters, drying out the hull, washing out the acids, filling in the voids with epoxy filler, and applying a new gelcoat or barrier coat.
To repair gelcoat blisters, you need to follow these steps:
Lift your boat out of the water and place it on stands or blocks.
Mark the locations of all visible blisters with a marker or tape.
Use a grinder or a chisel to remove the gelcoat and expose the blisters. Be careful not to damage the fibglass substrate.
Use a heat gun or a fan to dry out the hull for at least 48 hours. You can also use a moisture meter to check if the hull is dry enough.
Use a hose or a spray bottle to wash out the acids from the blister cavities with fresh water. Also, you can use baking soda or vinegar to neutralise the acids.
Let the hull dry again for at least 24 hours.
Mix the epoxy filler and hardener according to the manufacturer’s instructions.
Use a putty knife or a syringe to fill in the blister cavities with the epoxy filler. Make sure to level the filler with the surrounding gelcoat.
Wait for the epoxy filler to cure according to the manufacturer’s instructions.
Sand the filled areas with 80-grit sandpaper to smooth out any excess filler.
Wipe the hull with acetone or a similar solvent to remove any dust or grease.
Apply a new gelcoat or barrier coat over the repaired areas, following the same steps as described above.
Conclusion
Gelcoat blisters are a common problem that can affect any fibreglass boat. They are caused by water penetrating the gelcoat and reacting with the fibreglass resin, creating acidic by-products that form pockets of fluid under the gelcoat. Gelcoat blisters can damage the performance, appearance and value of your boat, and if left untreated, they can lead to more serious problems such as delamination and osmosis.
The best way to prevent gelcoat blisters is to keep your boat dry as much as possible, or to apply a barrier coat over the gelcoat. The best way to repair gelcoat blisters is to grind them out, dry out the hull, wash out the acids, fill in the voids with epoxy filler, and apply a new gelcoat or barrier coat.
By following these tips, you can master gelcoat blisters and enjoy your boat for many years to come.
Tom Crosby
MIDLANDS BOAT SURVEYS
NEWARK ON TRENT, NOTTINGHAMSHIRE, UNITED KINGDOM
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