Andrew Pace
Blisters are a common issue for boat owners, affecting the appearance and potentially the performance of their vessel. They occur when moisture gets trapped between the hull's outer surface and the underlying layers, causing the outer surface to lift and bulge. While they rarely threaten the structural integrity of a boat, they can be unsightly and detract from its resale value. If left untreated, blisters can cause more serious issues, such as boat pox, which requires specialist attention. However, with the right tools and techniques, boat owners can easily repair fiberglass blisters and restore their vessel to its original condition. This introduction will cover the causes of blisters, their potential consequences, and provide an overview of the repair process.
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What You'll Learn
Pop blisters and drain liquid
Blisters in fibreglass boats are a common occurrence, affecting one in every four fibreglass boats. While they rarely threaten the structural integrity of a boat, they can be unsightly and detract from its resale value. The liquid inside a blister is acidic and gushes out under pressure, so it is important to take the necessary precautions when popping the blisters.
To pop the blisters and drain the liquid, use a chisel or screwdriver. Hold the tool at arm's length and wear protective goggles to ensure your safety. The liquid will gush out under pressure, so be prepared to catch it with a suitable container. Once the liquid has been drained, use a disk grinder loaded with a 36-grit sanding disk to grind the open blister into a depression. Ensure that the depression is no deeper than the last damaged layer of laminate beneath the gelcoat. A good rule of thumb is to grind the depression 20 times wider than it is deep.
After grinding, tap around the blister pocket with the handle of a screwdriver or chisel to ensure that the surrounding area is also ground down. This will help create a smooth surface for the repair. Once the grinding is complete, it is important to clean the area thoroughly. Use an alcohol prep pad to wipe the cavity completely clean, changing the pad frequently to remove any contaminants.
If you are dry-storing your boat for the winter, it is recommended to grind and scrub the blisters at haulout but wait to fill them until launch time. Just before filling, scrub each depression briskly with a clean rag dampened with acetone. This will help prepare the surface for the repair.
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Grind the area
After popping the blisters and letting the liquid drain out, the next step is to grind the area. This is done to create a depression in the area where the blister was. It is important to ensure that the depression is no deeper than the last damaged layer of laminate beneath the gelcoat. A good rule of thumb is to grind the depression to be 20 times wider than it is deep.
To grind the area effectively, you can use a disk grinder loaded with a 36-grit sanding disk. This will help you shape the depression and smooth out the surface. Make sure to wear protective gear, such as goggles, during this process to ensure your safety.
It is also recommended to not only grind the open blister but also the surrounding areas. This helps ensure that any potential weak spots are addressed and treated. You can use the handle of a screwdriver or chisel to tap around the blister pocket, extending the grinding area as needed.
After grinding, the depression should be washed and dried thoroughly. This step is crucial to ensure that any remaining liquid or debris is removed before proceeding with the next steps of the repair process.
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Wash, dry and fill with resin
Once you've popped the blisters and ground the area, it's time to wash, dry, and fill the cavity with resin. Before applying the resin, it is important to ensure that the cavity is free of contaminants such as dust, existing paint, grease, or oil. Use a clean rag dampened with acetone to scrub the cavity briskly. You can also use an alcohol prep pad to wipe the cavity, changing the pad frequently to ensure a thorough clean.
After cleaning the cavity, you can begin the process of drying. You can use a heat gun or a hairdryer to speed up the drying process, but be cautious as excessive heat can ignite the resin. It is recommended to use a box fan to blow the fumes away from the heat source. Alternatively, you can use a small piece of towel loosely stuffed into the hole and soaked with acetone to draw out any remaining moisture as it dries. This method requires careful handling due to the acetone's ability to dissolve organics and penetrate the skin.
Now, you can prepare the resin mixture. For shallow blisters, mix a small amount of epoxy resin with a hardener, thickening it to a peanut butter consistency with colloidal silica. You can also use a filler like glass bubbles, fumed silica, or milled glass fibers to create a putty. Epoxy resin is the recommended choice due to its stronger adhesion and better water impermeability compared to polyester resin. Additionally, ensure that your resin is within its shelf life and has been stored properly.
Finally, fill the cavity with the thickened epoxy mixture using a squeegee to compress and fair the filler. Make sure to take your time to fair the epoxy while it is still wet, as silica-thickened epoxy can be challenging to sand down once dried. After filling and fairing, allow the resin to cure. The curing time will depend on the type of hardener used (fast, medium, or slow) and the working temperature. Ideally, work in temperatures around 75-80 degrees Fahrenheit.
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Apply epoxy and filler
When it comes to applying epoxy and filler to repair fiberglass blisters on a boat, there are several steps to follow. Firstly, it is important to ensure that the blister cavity is clean and free of any contaminants. This can be done by using an alcohol prep pad to wipe the cavity, changing the pad frequently to avoid reintroducing contaminants.
Next, you will need to prepare the epoxy mixture. For shallow blisters, a small amount of epoxy, around one or two pumps, should be thickened to a peanut butter-like consistency with a filler such as colloidal silica. It is important to avoid using microballoons or any hollow or absorbent materials, such as talc, as these will not provide a lasting treatment. The silica-thickened epoxy should be applied to the cavity using an acid brush, ensuring that the entire surface of the depression is covered. This step should be done carefully, as silica-thickened epoxy is difficult to sand down once it has dried.
For deeper blisters, you will need to cut a disk of fiberglass cloth to match the diameter of the depression, and then cut several more disks, each slightly smaller than the last. These disks will be layered into the epoxy mixture to create a strong repair. Once the epoxy mixture is ready, it should be applied to the blister cavity, ensuring that the cavity is completely filled. A squeegee can be used to compress and fair the filler, creating a smooth surface.
Finally, the repaired area should be left to dry. After it has dried, the area can be sanded down to create a smooth finish. It is important to note that if the blisters are extensive, a complete overhaul of the gelcoat and barrier coating may be necessary. Additionally, a barrier coat of epoxy can be applied to the hull to prevent future moisture penetration and protect the boat from blister formation.
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Sand and paint
After grinding the open blister into a depression, scrubbing the area with a solution of trisodium phosphate and hot water, and letting it dry, it's time to sand and paint. Before painting, it's important to prepare the surface properly to ensure a smooth and even finish. This involves sanding the area to create a firm edge and feathering the edges of the intact coatings. This process is known as "wet sanding" and is an important step to ensure the safety of the worker.
Once the area has been sanded, it's time to prime and apply the finish coatings. It is recommended to use a bottom paint finish to maintain the glossy exterior of the boat. This type of paint will not only provide a smooth and aesthetically pleasing finish but will also protect the boat from the elements. It is crucial to follow the instructions on the paint can and ensure that the area is well-ventilated during the painting process.
For shallow blisters, a small amount of fresh epoxy, thickened to a peanut butter consistency with colloidal silica, can be used to fill the depression. After filling, the area should be sanded again to level out the filled cavities. This process ensures that the repair is smooth and even with the contour of the hull. It is important to note that silica-thickened epoxy can be difficult to sand, so extra care should be taken to fair the epoxy while it is still wet.
After the repair is complete and the epoxy has cured, a final sanding may be necessary to ensure a smooth finish. This step will help to remove any excess paint or epoxy and create a seamless blend between the repair and the original surface. Once the sanding is complete, a final coat of paint can be applied to the entire area, restoring the boat to its original glossy finish.
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Frequently asked questions
Blisters on a fiberglass boat are caused by moisture getting trapped between the hull's outer surface and the underlying layers. This moisture causes the outer surface to lift and bulge.
Blisters rarely threaten the structural integrity of a boat. More than 95% of boats with fiberglass blisters do not develop any other issues. However, if left untreated, blisters can be dangerous.
You will need an appropriate quantity of epoxy resin and hardener, a filler to thicken the epoxy into a putty (colloidal silica), a quart of acetone, a box of TSP (trisodium phosphate), a few acid brushes, and a 36-grit sanding disk.
First, pop the blisters to let the liquid drain out. Then, grind the area to create a depression. Wash, dry, and fill this depression with resin before fairing and painting over the area.
To prevent blisters from forming, you need a completely waterproof boat. This can be achieved with a complete barrier coating of epoxy, but it can be quite expensive. Regular maintenance is a more cost-effective way to prevent and treat blisters.
