Fibreglass boat repair – letter box repair (Part I)

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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.

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Part II explains the technique of posting the backer through the hole and bonding it in place on the back of the panel.

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. 

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