That Soft Spot!!!
You hear it often. Maybe a surveyor told you, or you just felt it. A soft spot in a deck or hatch. It’s always worse than you think. The term “balsa core” Or “Cored layup” comes from the design and build where wood usually Balsa for it’s light weight (Remember the rubber band flyer planes as a kid?) anyway, The balsa is laid in between two thin layers of fiberglass to offer strength without adding too much weight. So So how does the soft spot happen? Simple, two ways:
Every time you drill and drive a screw, or install a snap for example? If it’s not properly sealed with a proper sealant, you open up the possibility of water intrusion. The second way is when a split, crack, or void (Airpocket in the layup becomes open. Once that moisture gets in between the two layers of glass, the balsa literally soaks it up like a sponge and turns to mush. Try this at home. Take a dry sponge and put water on one corner. What happens? It spreads far and wide and the sponge is now pliable. So you guessed it, when the core gets wet the two thin layers of glass have no more strength and you get a soft spot.
Can it be fixed? The short answer is yes but hang on to your wallet. It won’t be cheap! The long answer is it’s like filling a tooth. The area has to be cut open, all the old wet core removed, Glass dried out, and the rebuild process begins. (New core, Glass, Resin, and gelcoat for color.) It’s a labor intensive process that only a professional can do properly. Back to the tooth reference. The hatch in this picture had a soft spot the size of my foot. (About 12”x3”) Once the hatch was opened and the coring removed until you are at solid core? (Because you don't want to mate damp wood with dry or you have just started the sponge process again) The size is a little more than half the whole hatch itself. So now you know. Don’t drill holes or run screws and snaps without the proper sealant. and keep that core intact. I have literally seen really nice boats “totalled” because the cost of repair far exceeded the value of the boat itself!
I've done that repair on a previous boat, in many places. Old sealant is another danger. Even if your stanchions and window tracks were done perfectly, the old sealant will only last so long before it dries, shrinks, and cracks. Pulling all the fittings off and resealing, is the only way to preserve wood encapsulated in fiberglass.