Repairing a Rotting Wood Deck With Fiberglass on the Top

Remove all on-deck hardware that could obstruct the removal of the fiberglass; cleats, instruments, lights, sail tracks, toe rails and winches must all go until the entire unsound surface is unobstructed. If all the fasteners are marine grade, keep them with the components for reuse.

Make shallow pocket cuts around the area suspected to comprise rotted wooden core, using a circular saw. Make the cuts only deep enough to cut through the fiberglass skin, not into the wood beneath. Use a wide-bladed pry bar if necessary to separate the fiberglass from the substrate; dispose of the waste fiberglass panel. Enlarge the hole in small increments until sound wood is present, uninterrupted, along each side.

Remove the rotten wood until only sound wood remains. It is vital to the project to carry this out completely. Most decks are made using end-grain balsa wood or untreated ply for the core. Both these woods are particularly easy to identify as either sound or rotten; when rotten they change color to almost black and retain the moisture, so they always feel wet.

Prepare the exposed area for repair by first allowing any residual moisture to evaporate. Next, scour the entire inner surface of the hole with 80-grit sandpaper; the upper -- inside -- surface of the deck’s underside fiberglass skin should be smooth, clean and unobstructed when you finish the sanding. If you cannot sand the surface perfectly smooth, brush on a layer of resin and allow it to cure.

Round off the edge of the hole in the upper skin using an electric sander. Create a smooth, uniform 12:1 bevel around the entire hole. This graduated bevel allows sufficient contact between the new and the old fiberglass to permit a good bond.

Cut a replacement panel of new balsa or treated ply to the exact size and shape of the hole in the original core. If the deck is anything but flat, use “contour core” material -- available from most specialist boat stores and some marinas -- which features a series of perpendicular cuts, approximately 1 inch apart, that allow the wood to conform to curvatures of the inner skin.

Anchor the new wood in place using a proprietary core bedding mixture, observing all the manufacturer’s instructions and advisements. Once you locate and compress down the replacement panel, clean the area of all excess bedding mixture before it begins to set. It is imperative that you not compromise the 12:1 bevel by contamination. Allow the work to cure completely before moving on.

Install a new fiberglass upper skin in accordance with the structure of the boat. If you do not know the correct type of fiberglass mat and resin to use, consult with your supplier; equipped with the boat’s details, most specialty outlets and marinas with repair facilities should be able to offer direction and the appropriate supplies.

Use an orbital sander to remove any high spots and rough patches after the repair is fully cured. Follow up with finish treatments -- gel-coat, painting or however the remainder of the deck is finished -- as appropriate to the vessel. Replace the on-deck components, using new fasteners if the originals were not marine grade and in good condition.