Replacing Plywood On Your Boat: A Step-By-Step Guide

Replacing plywood on a boat is a common repair job for boat owners. The process can be straightforward, especially if you're simply patching a small area of damage. However, it's important to carefully assess the extent of the damage and decide whether a simple repair or a full restoration is needed. In some cases, it may be easier to replace an entire panel rather than attempting multiple small repairs. When replacing plywood, it's crucial to use the right type of wood and ensure proper bonding and sealing to prevent future water damage and rot.

Identify and cut out the damaged plywood

Before you start repairing your plywood boat, it's important to decide whether the job is a repair or a restoration. If you want to get the boat on the water quickly, don't take it completely apart.

The first step is to identify and cut out the damaged plywood. It is normally easier to replace the whole sheet rather than patch it. If you do need to patch it, mark out and cut the offending area along straight lines, as this will make it easier to cut a replacement patch. A square or rectangle will be easier to cut and replace.

The ply will likely be epoxied to the frames, stringers, and bulkheads. Cut around these and then remove the glued-on ply. As it is easier to cut from the outside of the hull, mark the positions of the frames on the surface. From the inside, drill a few fine holes along with the framing so they can be seen from the outside, then draw a line to join them up.

To avoid cutting through or damaging any frames, stringers, or bulkheads, use a skilsaw or router with the depth set to the hull thickness. If the boat has been skinned with fibreglass, you will need to use a carbide bit in the router.

The ideal patch would be one with bevelled (scarfed) sides, which would fit perfectly into a bevelled hole. However, it is much easier to get a good fit if the hole and patch are cut with edges at right angles to the surface. You will probably need to use a chisel or Japanese saw to finish and trim the corners.

If the damage was caused by rot, consider treating the surrounding wood to kill off any remaining rot spores.

Prepare the replacement patch

The replacement patch should be made from the same type of plywood as the original. Cut the patch to match the outside dimensions of the bevel. You can use a circular or reciprocating saw to cut out the smallest square or rectangle that encloses all of the damage. Don't forget to check the inside of the panel to make sure you've removed the full extent of the damage.

Next, grind a scarf on the edges of the opening to create a stronger bond. This can be done using a disc sander, sanding a minimum of an 8:1 bevel back from the edge of the opening (8 times the thickness of the wood).

Use a disc sander to create a bevel on the inside edge of the replacement panel, which should match the bevel on the opening. Then, dry fit the new panel so that it sits just slightly below the existing surface.

Before bonding the panel, wet out the bonding area with a resin/hardener mixture and recoating the end grain as necessary. Keep in mind that the plywood will absorb a lot of the epoxy. Apply a mix of epoxy thickened with microfibres to the bonding area. The mixture should have the consistency of mayonnaise to form the most effective bond. You will need enough so that a small amount squeezes out when you push the new panel into position.

Cut and bevel the replacement patch

Cutting out the rot is the first step in repairing plywood on a boat. The offending area should be marked out and cut along straight lines, as this will make it easier to cut a replacement patch. The shape of the cut doesn't matter, but a square or rectangle is easier to work with. It is common for the ply to be epoxied to the frames, stringers, and bulkheads, so you will need to cut around these and remove the glued-on ply. The positions of the frames will need to be marked on the surface so that you can cut from the outside of the hull and avoid damaging any of these components.

The ideal patch will have bevelled (scarfed) sides that fit into a bevelled hole. While this is difficult to execute, it will result in a better fit. It is easier to cut the hole and patch with edges at right angles to the surface. A chisel or Japanese saw will be needed to finish and trim the corners.

If the boat has been skinned with fibreglass, a carbide bit will be needed for the router. If the damage was caused by rot, the surrounding wood should be treated to kill off any remaining rot spores.

The patch should be the same thickness as the original plywood. Create a template of the cut-out to use as a pattern for the patch. Cut the patch so that it fits the hole perfectly and lies flush with the hull's surface. The patch will need to be supported with but blocks. Before assembly, the patch and backing blocks should be given several sealing coats of epoxy.

Seal and glue the replacement patch

To seal and glue the replacement patch, start by wetting the bonding area of the patch with a resin/hardener mixture. You will need to recoating the end grain as necessary. Plywood will absorb a lot of epoxy, so use a generous amount.

Next, apply a mixture of epoxy and thickener to the bonding area. The mixture should be similar in consistency to mayonnaise. You will need enough so that a small amount is squeezed out when you push the new patch into position.

Now, clamp the new patch into position using temporary staples or screws. Clean up any excess epoxy before it cures. Once it has cured, remove the clamps and you will be ready to add the final layer of epoxy/filler.

Start by sanding the surface carefully. Then, use a spreader to fill low areas with an epoxy/low-density filler mix, which should be similar in consistency to peanut butter. Aim to slightly overfill low areas on the surface. Allow the mixture to cure before fairing.

Before gluing the patch, the wood will require proper preparation. A saw blade turned on its side can be used to scrape gashes into the wood to help create a good mechanical bond. You may need to get out your right-angle sander/grinder with some 24 grit and take everything back to bare wood. Then, reseal all the wood with epoxy. If any of the wood has been contaminated with oil or grease, this will have to be thoroughly degreased. If the oil has soaked in, it will require repeated washing with degreaser and solvent over enough time to allow the oil to come to the surface. Warming the wood might help bring the oil to the surface and fine sawdust will help to absorb it.

If you are replacing large areas on a small boat, the boat will need checking for any twist before gluing up. This can be done by using a straight edge and eyeballing to make sure everything is true.

Sand and fill the replacement patch

Sanding and filling the replacement patch is a crucial step in repairing plywood boats. Here is a detailed guide on how to do it:

Sanding the Replacement Patch:

• After cutting out the damaged area and inserting the replacement patch, it's time to sand the surrounding area.
• Use a belt sander to sand the inside surrounding edge of the hole and the exposed frames to remove any remaining glue and smooth out the edges.
• Take the wood back to its bare state. Use a right-angle sander/grinder with 24-grit sandpaper to ensure all the wood is sanded evenly.
• If the wood has been contaminated with oil or grease, it must be thoroughly degreased. Use a degreaser and solvent to wash the area repeatedly, allowing time for the oil to come to the surface.
• Warming the wood or using fine sawdust can help absorb the oil. Ensure all the oil is removed before gluing.
• Check the boat for any twists or warping, especially if you're replacing large areas on a small boat. Use a straight edge and eyeballing to ensure everything is true and aligned.

Filling the Replacement Patch:

• Before gluing the patch, prepare the wood properly. Use a saw blade turned on its side to create gashes in the wood, which will help create a strong mechanical bond.
• Seal all the wood with epoxy, coating the end grain as necessary.
• Apply a resin/hardener mixture to the bonding area, recoating as needed. The plywood will absorb a lot of epoxy, so ensure you have enough.
• Clamp the new panel into position using temporary staples or screws. Clean up any excess epoxy before it cures.
• Once the epoxy has cured, remove the clamps and sand the surface carefully.
• Mix an epoxy/low-density filler to a peanut butter consistency.
• Use a spreader to fill any low areas or gaps on the surface, slightly overfilling them.
• Allow the filler to cure thoroughly before proceeding to the next step, such as fairing or finishing.

Remember to work in a well-ventilated area when using epoxy and always follow the manufacturer's instructions and safety guidelines.

Frequently asked questions