Rick Conner of Southport, Florida, asks:
"My 1982 sloop has a keel–stepped rig and the balsa-cored deck around the mast partners is saturated with water. Can I drill holes into the deck and replace the core from the top using WEST System epoxy to make the repair? Or should I remove the wet balsa core from around the mast and fill the area with solid fiberglass? What about other spots in the deck that have the same problem?"
Don Casey replies:
To plan the best repair you must first understand that the function of a balsa core is to add depth to a structure, much like the central web of an I-beam. The thin skins on either side of the balsa core lack adequate stiffness unless they are attached to either side of the core material. Attached is the operative word here. If the skins are separated from the core, which is what happens when wood core gets wet, the three components of the composite structure will slide across each other when loaded, acting more like a leaf spring than an I-beam.
Instead of a balsa, plywood or foam core, the two skins might be separated by a mostly hollow grid structure or even one that has parallel perpendicular strips firmly attached to both skins. These “cores” are primarily air-filled but they are as rigid as a more solid wood or foam core and are less susceptible to water damage. But because of the fractional amount of contact area, the bond between skin and core will be less robust, which is why most builders prefer solid core materials.
You can create support columns within a damaged core by drilling or sawing holes through both the upper skin and the punky balsa core and filling these with reinforced epoxy resin. For such a repair to be successful, the epoxy columns must be large enough to bond with both the top and bottom skins. They must also be close enough together to adequately support the span between columns.
This method is most appropriate when a relatively small area is involved. To repair a large area you should remove the top skin, replace the damaged core and then relaminate a new skin on top of it. In fact, replacing the core is probably less work and will produce a better result. In my view, any builder who leaves the core material exposed at the mast partners should be ashamed. But now you may be able to use this failing to your advantage.
First dig out the wet core as far back from the partners as is feasible. Vacuum out the debris, then carefully sand the interior surface of both skins with 36-grit sandpaper and wipe away all the sanding dust with acetone. Fill the void with epoxy thickened with colloidal silica. Use either an epoxy syringe and plastic straw or a length of tubing to inject the resin into the back of the void. Inject in small batches and, to avoid overheating as the epoxy sets, let previous injections “kick” before you add more. Make the inside edge flush with the skins around the partners and that should fix the core problem you have around the mast.
As far as other damage is concerned, flexible decks are epidemic in older boats and the efficacy of any repair depends on the value of the boat and the kind of sailing you plan to do. In most cases, however, a well executed do-it-yourself core repair almost always pays for itself in terms of resale value.