Kristian Ferrell
Owning a fibreglass boat can be a lot of work, especially when it comes to repairing screw holes. Whether you're refitting, resizing, removing obsolete equipment, or fixing mistakes, it's important to know how to plug screw holes properly to avoid water intrusion and ensure a strong, long-lasting repair. While some jobs may require a professional boat builder, plugging a screw hole or two is something you can handle yourself with the right tools and knowledge. In this article, we will explore the different methods and materials for plugging screw holes in fibreglass boats, including epoxy, gel coat, and wood plugs, to help you get your boat back in tip-top shape.
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What You'll Learn
Using epoxy to plug the holes
When repairing machined holes in a fibreglass boat, the strategy depends on the size, purpose, and location of the hole. For instance, whether the hole is below the waterline or not will determine the best approach. It's important to ensure that the repair is strong enough for the anticipated worst-case load and to consider the costs in time and money, as well as the skill required.
If you're using epoxy to plug the holes, there are several methods you can use. One method is to first drill out the hole to make it oversized, then fill it with thickened epoxy, and finally, redrill for mounting screws. This method can be used with products like Marine Tex, a thickened epoxy sold in small tubes, or 3M 5200, an adhesive sealant that cures into a hard, water-tight, and super-strong material. When using 3M 5200, be sure to use both the screw-dip and squeeze methods to get the product into the holes.
Another approach is to use a finger file to grind a one-inch square around each hole, then lay in a small amount of fibreglass polyester or vinyl ester. After it cures, sand and fill with 3M body filler, and then apply a matching gel coat. This method ensures that the repair has strength and won't crack, shrink, or fall out over time. Additionally, if you're working on a plywood core, you can sand the inside of the hole to expose clean wood, cut a wood plug that matches the thickness of the plywood core, and glue it in place with epoxy thickened with 403 Microfibers.
Before applying epoxy, it's crucial to prepare the bonding surfaces. Clean the surfaces with an appropriate solvent to remove any contaminants, and use a heat gun or hairdryer to ensure they are thoroughly dry. For non-porous surfaces, sand with 80-grit paper to provide a texture the epoxy can "key" into. If you're unable to access the backside of the hole, you can simply wet out the hole with neat epoxy to create a strong bond and prevent water intrusion.
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Sanding and filling with fibreglass cloth
When repairing screw holes in a fibreglass boat, the strategy will depend on the size, purpose, and location of the hole. It is important to determine whether the wood is rotten and needs to be replaced. If the hole is below the waterline, it is crucial to ensure that the repair is strong enough to withstand the worst-case load.
Before beginning the repair, it is essential to clean the surfaces with an appropriate solvent to remove any contaminants such as oil, grease, or wax. This will ensure a good adhesion for the repair. If you are using epoxy, make sure the bonding surfaces are completely dry. For non-porous surfaces, sand with 80-grit paper to provide a texture for the epoxy to "key" into.
If the hole is larger than 2 inches in diameter, you may need to cut a wood plug that matches the thickness of the original plywood core. Sand the inside of the hole and the perimeter to create a slight chamfer that flares out onto the fibreglass. Dry fit the plug to ensure a loose fit, then remove it and glue it in place with epoxy thickened with microfibres. Allow it to cure to a soft gel before filling any low spots with a layer or two of fibreglass cloth.
For smaller holes, you can fill them with epoxy thickened with colloidal silica or another high-density filler. First, sand the inside of the hole and the surrounding area to expose clean laminate. Apply a coat of unthickened epoxy to these areas and to a piece of fibreglass cloth. Allow it to cure, then fill the hole with the thickened epoxy and sand back to level once cured.
If you are unable to access the back of the hole, you can use a syringe to fill it from the bottom to ensure that all air is forced out of the hole. Use a soda straw to extend the length of the syringe if needed. After filling, clean up around the hole to minimise sanding time.
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Using gel coat repair
When repairing screw holes in a fibreglass boat, the correct strategy will depend on the size, purpose, and location of the hole. For instance, if the hole is below the waterline, it will need to be sealed with epoxy before being refilled with fibreglass resin and gel coat.
If you are using gel coat repair, the first step is to ensure the bonding area is clean and dry. Use a drill bit to remove any remaining compound inside the hole and a razor to remove any compound on the surface. Clean the area with a solvent to remove oil, grease, wax, sealants, or other contaminants. You can use a heat gun or hairdryer to ensure the surface is completely dry. If the surface has contaminants before you sand, they will get sanded into the gel coat. Sand the whole area with 220-grit paper on a block, taking care not to gouge. When you're finished, rinse the area with water and dry it with clean paper towels.
Once the area is prepared, you can begin the gel coat repair. Tape the underside of the hole and use a putty knife to push in the filler. Mix the filler according to the instructions and use a plastic spatula to mix in the hardener. Apply the filler to the hole and let it cure. Once it is dry, sand it with 1200-grit sandpaper and use a buffing compound over the area.
If you are repairing a large hole, you may need to use a wooden dowel plug to fill the space. Seal the plug with epoxy and then epoxy it into the hole before applying the gel coat. If you are repairing a small hole, you can use a gel coat patch kit. Make sure the gel coat colour matches the boat, although it doesn't have to be exact.
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Drilling out holes and redrilling
Drilling out the holes and redrilling is a common method for repairing screw holes in fibreglass boats. This method is effective for plugging old screw holes and can be done without involving a boat builder. It is important to note that working on holes below the waterline requires extra caution to prevent water intrusion and exposure of the hull or transom coring to water.
The first step in the process is to determine the size, purpose, and location of the hole. This will help you choose the appropriate repair strategy, ensuring it is safe, adequate, and realistic. If the holes are below the waterline, it is crucial to use materials that are watertight and strong, such as 3M 5200 adhesive sealant.
Next, it is recommended to countersink both sides of the holes if possible. This creates flared ends that provide a more secure plug. Taping the back of the holes can also be done at this stage. It is important to ensure that the bonding areas are clean and dry before proceeding. Surfaces should be cleaned with a suitable solvent to remove any contaminants such as oil, grease, or wax.
After preparing the holes, you can begin the two-step bonding process. First, wet out the hole with neat epoxy, ensuring a strong bond to prevent water intrusion. Then, drill out the holes slightly larger to remove any old sealant. This can be done using a die grinder or a similar tool to clean and expose the raw glass.
Once the holes are prepared, you can fill them with thickened epoxy. Marine Tex, West System epoxy repair kits, and 3M 5200 are commonly recommended products for this purpose. After the epoxy has cured, you can redrill the holes for mounting screws. If the screws will be below the waterline, it is recommended to use 5200 on the threads of the screws for added sealing.
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Using plastic plugs
When repairing screw holes in a fibreglass boat, the strategy will depend on the size, purpose, and location of the hole. One option is to use plastic plugs, which can be purchased from hardware stores such as Home Depot. Here is a step-by-step guide on how to use plastic plugs to repair screw holes in a fibreglass boat:
First, ensure the bonding areas are clean and dry. Use a suitable solvent to remove any oil, grease, wax, or other contaminants. A heat gun or hairdryer can assist in the drying process. If the surface is non-porous, sand it with 80-grit paper to provide texture for the epoxy to "key" into.
Next, you can apply the plastic plugs. For smaller holes, you may need to use silicone or epoxy in addition to the plugs for a secure fit. The plugs should be pushed into the holes and allowed to cure. If needed, you can tape over the holes from the outside while the plugs cure. Once cured, you can remove the tape and, if necessary, sand the area to ensure a smooth finish.
If the holes are larger, you may need to seal the exposed surfaces inside the holes first. Allow the sealer to dry for a couple of days, then coat the inside surfaces with a slow-set epoxy. This will soak into the coring and create a strong bond. Once the epoxy is sealed, you can fill the hole with a plastic plug.
After the plastic plugs have been inserted and sealed, you may need to apply a final top coat. This can be done with a gel coat or paint that matches the surrounding surface. Remember to add a small percentage of wax to the top coat to ensure proper air curing.
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Frequently asked questions
You can use a product like Marine Tex or 3M 5200 to fill the holes. For larger holes, you can cover them with a clamshell vent.
The best way to plug screw holes in a fibreglass boat is to use the right screws for the job. Manufacturer-supplied hardware may not be up to the task. It is best to source and supply the hardware yourself.
A quick and easy way to plug screw holes is to use plastic plugs, available at hardware stores like Home Depot. You can also use silicone and simply replace the screws without the bracket to fill the holes.
