Cure Moisture in a Balsa Core

Vespera had a glued teak deck. Originally about 2000 tiny retainer screws were used to hold the planks in place while the glue cured. Some of those screws eventually leaked allowing moisture to get into about 40% of the deck.

Note, Vespera’s deck was very sound structurally. The only rot in the deck was localized around the through deck fittings. Here the core was removed. See the next section on balsa core rot.

To fix the decks moisture problem small holes were drilled in back of the core. After the core was dried, it was sealed by injecting polyurethane sealant into the core. This was especially needed because the core was covered by a head liner that could trap humidity. Because air could get into the core after we drilled holes into it, mold could form inside the core. And mold would eventually dissolve the bond between the balsa and the fiberglass.

Because polyurethane sets up in moisture and takes weeks to cure inside the core, the sealant not only seals the core it works its way into voids and forms very good bonds.

The tip of an automotive needle point grease fitting was drilled out so that caulk could be easily pumped through it.

We used a moisture meter to find moisture problems in the balsa core and mapped out the moisture variations into Moist, High-Moist and Wet. From the inside, we drilled 1/8” holes in a 1”x 1” grid and allowed the laminate to dry out. The laminate in the picture was High-Moist when first tested. After going through 1 Minnesota winter it tested Dry. The real drying occurred when outside temperature moved above and below freezing. Moisture meter tests during that period showed a dramatic change while tests taken when the temperature was steadily above freezing showed little change.

Here self sticking kitchen shelf paper with a checker design was used as a pattern to neatly drill holes in a 1” x 1” grid.

Here the cardboard cartridge from a small hand held grease gun was being filled from a full size tube of sealant. Of course grease from the small cartridge was cleaned out first. And a ¼” hole was drilled in to cartridge cap. Note, an inexpensive sealant is being used because this balsa core was structurally strong and above the water line.


Stan Sroga



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