Andrew Pace
Spraying gelcoat on a fibreglass boat is a messy and challenging task, but it is essential to maintaining your boat's appearance and seaworthiness. A well-applied gelcoat can last ten years or longer, protecting the fibreglass surface and acting as a barrier to water absorption. Before spraying, it is important to prepare the surface by removing any dirt and debris, and old gelcoat, and sanding the area to be coated. It is also crucial to choose the right day and conditions, avoiding direct sunlight and extreme temperatures. Once the surface is prepared, you can apply the gelcoat with a spray gun, being careful to avoid overspray and creating a smooth, even finish.
Spraying Gelcoat on a Fiberglass Boat
| Characteristics | Values |
|---|---|
| Purpose | To maintain the boat's look and sea-worthiness |
| Protection | Acts as a barrier to water absorption and protects the fiberglass surface |
| Surface Preparation | Remove dirt and debris, sand the entire area to be gelcoated with 80 to 120 grit, and tape off areas that should not be in contact with the gelcoat |
| Temperature | Avoid applying in temperatures below 60°F or above 80°F |
| Ventilation | Ensure adequate ventilation and avoid enclosed spaces |
| Application Method | Use a spray gun, roller, or brush |
| Additives | Consider using a wax surfacing solution in the final coat |
| Catalyzing | Catalyzing is crucial and complicated; have all supplies ready before mixing |
| Cleanup | Use boat soap to gently wash away dirt and grime |
| Polishing | Use marine boat polish with carnauba wax to protect the gelcoat and add shine |
| Curing Time | Touch-cure time is 2-3 hours; full cure takes 7 days under normal conditions |
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What You'll Learn
Sanding the fiberglass substrate
Before you begin sanding, it is important to prepare the surface. If you are working with a new fiberglass component, it will have a thin layer of gelcoat on its surface. Allow the fiberglass to cure in the sun for 2 to 7 days to cure this initial layer of gelcoat. This process helps release any air pockets in the gelcoat, preventing issues during sanding and painting.
Once the fiberglass is cured, assemble all the components, if applicable. For example, if you are working on a boat, assemble the body, doors, and other parts before sanding. This allows you to sand continuously between each component, creating a smooth and consistent finish.
Next, clean the entire fiberglass surface with a wax and grease remover. This step is crucial, especially when working with new fiberglass, as it removes the release agent used during the manufacturing process. Purchase a suitable wax and grease remover from an automotive parts store.
Now, you can begin the sanding process. For the initial sanding pass, use coarse-grit sandpaper, such as 80 or 100-grit sandpaper. Remember to cross-hatch your sanding. Start from the top right corner and move to the bottom left corner, then switch directions and go from the top left corner to the bottom right corner. This technique helps achieve a consistent finish.
During the sanding process, be careful not to sand all the way through the gelcoat into the fiberglass. Over-sanding can weaken the component and create holes in the fiberglass, leading to paint rupture later on. As a guide, sand the gelcoat until it loses its shine. When the entire component appears dull, you have sanded enough for the primer or paint to adhere properly.
After sanding, fill any low spots or imperfections in the fiberglass with a putty. Use a fiberglass glazing putty and work it into the low spots until it is flush with the rest of the surface. Once the putty is dry, sand it down gently until the surface is smooth and even.
Finally, wipe away any dust from sanding with a dry rag. Apply a rubbing compound to the fiberglass using a cloth and buff it in a circular motion. The rubbing compound will help smooth out the surface and create a shinier finish. If you are working with a large surface area, an electric buffer can be used to speed up this process.
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Using a spray gun
When using a spray gun to apply gelcoat to your fibreglass boat, there are several important steps to follow. Firstly, ensure the surface is properly prepared. Remove any old gelcoat from the hull, especially if it is blistered, using a power sander to get back to the fibreglass. If the old gelcoat is mostly intact but simply faded, you can apply the new coat directly on top.
Next, check the weather. Avoid applying gelcoat in direct sunlight, extreme temperatures (below 60°F or above 80°F), or in an enclosed space. Choose a well-ventilated area and cover or remove any parts of the boat you don't want to come into contact with the gelcoat, such as cleats, ropes, and seals. Use duct tape for a better seal.
Now you're ready to start spraying. Choose the right gelcoat for your project—with or without wax—and prepare it according to the manufacturer's instructions. Make sure you have all the necessary parts and supplies ready before you begin, as once you start mixing, the clock is ticking. Set up your spray gun and air compressor if it doesn't have one built-in.
Follow the manufacturer's instructions for spraying technique and nozzle settings. Apply the gelcoat in thin, even coats, being careful not to miss any spots. Check for any bubbles or blisters, as these are weak points that can allow water absorption. Treat them immediately if found. Allow the gelcoat to cure completely before using your boat.
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Avoiding direct sunlight
When spraying gelcoat on a fibreglass boat, it is important to avoid direct sunlight. This is because ultraviolet (UV) light degrades and destroys the wax, which will affect the longevity of your boat's gelcoat.
To avoid direct sunlight, choose your day carefully and watch the weather. Avoid applying gelcoat in temperatures below 60°F or above 80°F. If you store your boat on a trailer, try to park it in a shaded area, away from trees, and minimise direct sunlight. You can park next to a building or tall fence to achieve this.
Additionally, when the gelcoat is curing, do not store the boat in direct sunlight. Keep it under cover, either under canvas or in a boathouse, to protect its finish from the sun.
Finally, horizontal surfaces, such as decks, have the most exposure to sunlight. Avoid applying wax to these walking surfaces as it will make them slippery and unsafe. Instead, scrub these areas often to remove dust and dirt that can be ground into the gelcoat.
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Checking for bubbles
When spraying gelcoat on a fiberglass boat, it is essential to check for bubbles and blisters and treat them immediately. Bubbles are the weakest points in the coating and can lead to water absorption into the fiberglass hull, compromising the boat's longevity and seaworthiness.
To check for bubbles, carefully inspect the surface of the boat, especially below the water line, as this area is most susceptible to water absorption. Look for any signs of popping or raised areas on the gelcoat surface. Bubbles can occur due to trapped water underneath the gelcoat or moisture permeating into the hull structure.
If you find bubbles or blisters, it is crucial to address them promptly to prevent further damage. There are several ways to deal with bubbles in gelcoat:
- Rip off the affected gelcoat and reapply a new layer.
- Remove the gelcoat and paint the area.
- Grind the bubbles down and fill them with epoxy, then apply several coats of a barrier coat.
- Leave the boat out of the water and let it dry with a fan blowing air at the bubbles until they disappear, then apply a barrier coat.
- Take the boat to the manufacturer to apply a blister-proof gelcoat.
- Keep the boat on a trailer, and the bubbles should eventually disappear.
- Apply dry ice to the affected area until the bubbles go away.
It is important to note that leaving blisters untreated can lead to deterioration and may result in a more extensive fiberglass boat repair in the future. Therefore, it is advisable to address them promptly and monitor the hull's condition regularly.
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Applying multiple coats
Firstly, it is important to prepare the surface of the boat. Sand the entire area that will be gelcoated using 80 to 120 grit sandpaper. This process ensures the surface is smooth and free of any dirt, debris, or old gelcoat. If the old gelcoat is severely blistered, it is advisable to use a power sander to remove it and expose the fibreglass underneath. However, if the old gelcoat is only slightly faded, you can apply the new coat directly on top, allowing the new coat to harden effectively.
Next, clean the sanded area to remove any residual dust or particles. This step is crucial as it ensures that the new gelcoat will adhere properly to the surface. Use a solvent cleaner or a damp cloth to wipe down the surface, ensuring no fibres are left behind.
Once the surface is prepared, it is time to mix the gelcoat. Depending on the product, you may need to catalyze it before application. Refer to the manufacturer's instructions for the correct mixing ratio and technique. It is important to work in a well-ventilated area and wear protective gear, including gloves and eye protection, during this process.
Before spraying, cover any areas adjacent to the target surface that you cannot or do not want to remove. Use masking tape and masking paper to protect these areas from overspray. Additionally, if there are multiple coats to be applied, it is recommended to add a wax surfacing solution to the final coat of gelcoat to prevent webbing.
Now, you are ready to begin spraying. Use a spray gun to apply the gelcoat, ensuring you follow the manufacturer's instructions for nozzle settings and technique. Work systematically, maintaining a steady hand and speed to achieve an even coat. Allow the gelcoat to dry according to the manufacturer's instructions before applying subsequent coats.
Finally, inspect the boat for any bubbles or blisters, which are the weakest points of the coating and can lead to water absorption. Treat these areas immediately to ensure the longevity of your boat's finish.
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Frequently asked questions
Before applying the gelcoat, the surface of the boat should be free from dirt and debris. Sand the entire area to be gelcoated with 80 to 120 grit, and then remove any dust with a solvent cleaner. Tape around portholes and anything that you don't want in contact with the gelcoat.
Avoid applying gelcoat in direct sunlight or in temperatures below 60°F or above 80°F. Allow for ventilation and avoid enclosed spaces.
Gelcoat can be applied with a spray gun, or with a roller or brush for smaller jobs. When using a spray gun, be mindful of overspray, which can get on non-target areas.
To keep your fiberglass boat looking its best, wash it regularly with boat soap to remove dirt and grime. Use chamois cleaning cloths for a scratch and lint-free wash. Protect the gelcoat with marine boat polish containing carnauba wax to shield it from algae, UV rays, and saltwater.
