Soft deck – moisture in the sandwich core and how to repair it

Short answer

The deck and cabin top on many GRP boats are built as a sandwich: two thin fibreglass skins with balsa or foam in between. If a fitting leaks, water gets into the core. The deck goes soft, flexes when you walk on it and sounds dull when you tap it. If the core is wet but intact you can drill, dry and inject epoxy. If the balsa has rotted, the core must be replaced.

Find the damage

• Tap with a handle or a small hammer. A sound deck sounds crisp; a wet or delaminated one sounds dull.
• Walk and feel. Does the deck flex or creak around a cleat, a stanchion or a chainplate?
• Look for rust streaks, cracks around fittings and water dripping inside the boat.
• A moisture meter shows where the moisture is spreading. Compare with dry areas of the deck.

West System notes that PVC foam tolerates moisture better than balsa, but both can delaminate from the skins.

Dry and inject (minor damage)

This method works when the core is wet but not rotten, and the area is small.

1. Drill a pattern of holes through the top skin into the core, not through the bottom skin.
2. Let air and heat dry the core. West System warns: do not heat the surface above 50 °C, as this can damage the core and laminate.
3. Drill a few test holes between the old ones to check that the core is dry. Let everything cool to room temperature.
4. Inject epoxy thickened to a ketchup consistency with a syringe, one hole at a time until it comes out of the next.
5. Press the skins together with weights or screws until the epoxy has cured. Sand and fill the holes.

Replacing the core (large or rotten damage)

Blackened, crumbling balsa can't be dried back to health. The top skin is cut open, the old core is dug out and new core is bonded in with epoxy. The top skin is put back and the joints are laminated with a bevel, following the same principle as in repairing a hole in a fibreglass boat. It's a big job but you can do it yourself on a small area. If it's several square metres, or the mast step on a sailboat, it's a job for a boatyard.

Protecting the core at screw holes

How to remove and rebed a fitting is covered in sealing leaking fittings. Do this with the holes before the fitting goes back on, so water can't reach the core again:

1. Drill the screw hole oversize and dig out some core around the hole.
2. Fill with thickened epoxy and let it cure.
3. Drill a new hole in the cured epoxy, sized for the screw.

Safety

Epoxy can cause allergies. Wear nitrile gloves and safety glasses. Don't breathe in sanding dust from fibreglass and old balsa, so wear a respirator with a particle filter. Heat gently with a heat gun or lamp, never with an open flame against the core.

Frequently asked questions