How To: Fibreglass boat repair – letter box repair
The most important thing to do when you encounter a hole in your fibreglass boat is to assess the damage thoroughly. If the damage to the skin is larger than 10mm it’s best practice to use a backing to support the wet fabric patch until the epoxy cures.
Creating support at the back of a damaged panel is generally not a problem if there is access to the inside. However, if the boat has a fibreglass liner or if the back of the hole is inaccessible, an alternative method needs to be used – bonding a thin panel to the back of your repair. One such method is what we call a ‘letter box repair’ – so called because it involves ‘posting’ a backing panel through the damaged area and securing it with epoxy.
Although the installation method and backing material can be altered depending on the size and curve of the repair area, the method described below can accommodate the widest range of openings.
Follow the steps below to find out how to create a backing plate to match your fibreglass boat repair.
- Mask and wax the surface
Select and tape an area of the panel next to and several centimetres larger than the hole opening, to act as a mould for the layup of the backing. Mask the area outside the tape to protect from epoxy spillage. Apply a liberal coat of automobile paste wax or release agent to the mould area. The selected area should match the curve or contour of the repair area. - Prepare the glass fabric for bonding
Cut two pieces of 200g/m² glass fabric to the size of the waxed area. Add one piece of cloth for every 300mm increase in a hole diameter over 300mm. Lay the cloth on a plastic protected work table.
Wet-out the layers of fabric with WEST SYSTEM 105 Epoxy Resin®. Pour a small amount of resin/hardener mix in the middle of the cloth. Use a squeegee to spread the epoxy mix over the cloth until both layers are completely saturated. - Apply the fabric
Place the wet-out fabric against the waxed panel. Use a squeegee to smooth the cloth against the panel – always add a layer of peel ply as this will give a ready to bond to surface and protect the laminate from moisture, if working outside. The paste wax will prevent the cloth from bonding to the panel and allow it to be released. It should be flexible enough to conform to a moderately curved panel. Allow the backer to cure thoroughly. - Remove the backing plate
Remove the backing plate from the panel. Using a pallet knife or mixing stick, trim the laminated backer to the shape of the hole, 10mm larger than the hole on all sides. Next, drill three or four holes in the laminate and thread a length of wire through the holes, this way the wire can be bent to keep the plate in place while it cures.
Now that you’ve prepared your backing plate, it’s time to put it into place. In the second part of this Trade Secret, find out how to bond your backing plate securely, giving you the support you need to carry out your fibreglass boat repair.
Creating a solid backing plate is an essential part of any large structural repair to a fibreglass boat. Once your backing plate has cured in place you will be ready to apply layers of glass fabric to your repair, creating a resilient and aesthetically pleasing fibreglass boat repair.
In Part I, we described how to create a backing plate using your existing panel as a mould. Part II of this Trade Secret will detail the best method for bonding this backer using WEST SYSTEM® epoxy.
Follow the steps below to bond your backing plate to your repair.
- Sand the surfaces
Prepare the inside of the opening for bonding. Reach through the opening and thoroughly abrade the inside edge of the hole with 50-grit sandpaper. If peel ply has been used on the backing plate then, before bonding the backing plate in place, remove the peel ply.
- Pass the backing plate through the hole
Bend the laminate slightly so that it can pass through the opening, using the wire to keep it from dropping. If possible, grind the hole and trim the backer to an oval shape rather than a round shape. An oval shape will allow the narrower dimension of the oversized backer to pass through the wider dimension of the smaller hole without having to flex the backer.
- Apply the epoxy
Bond the backer in place using WEST SYSTEM Epoxy thickened with 404 High Density Filler. Apply adhesive to the inside edge of the hole and to the edge of the laminate.
- Pull into position
Using the wire for grip, pull the backer into position on the inside of the panel and fold the wire to keep the backing plate in place. Keep enough tension on the wire to hold the backer in position and bend the backer as necessary to match the contour of the panel. Some of the epoxy mix should squeeze out of the joint.
- Clean up the repair
Scrape away the excess epoxy mix and smooth the joint before it begins to cure. Clean this carefully with a glue brush or mixing stick to ensure it is smooth and will allow the repair laminate to drape into place. Allow the epoxy to cure thoroughly before removing the wire.
To find out more about the techniques used in these Trade Secrets, visit the WSI support page.
Coming soon to epoxycraft: fibreglass boat repair ‘how-to’ videos. Keep a close eye on our YouTube channel and epoxycraft for new GRP support videos.
Like epoxycraft? Love videos? Coming soon to epoxycraft, the latest in boatbuilding technical support, GRP repair videos. Watch out for our in-depth videos on how to prepare and conduct your very own fibreglass boat repair.
