Cure Balsa Core Rot problems

Note, Vespera did not have a rotten deck core except for small areas surrounding the through hole fittings. However, we will discuss a theoretical fix for a rotted deck at the end of this section.

All of Vespera’s through deck fittings had rot surrounding them. Some as little as a 1 inch ring of rot and some as much as 16 inches in diameter. Here, the core had to be removed without destroying the laminate. Then polyurethane was injected to replace the balsa.

In the picture below, about 6 inches of balsa core was removed using a flexible routing tool that we made for an electric drill. After the core was removed, the large hole was temporarily plugged by wrapping plastic bags around the original through deck fitting and forcing it back into the hole. Then 3M 5200 was pumped into each one of the three visible holes. The caulk was pumped directly from the caulk gun because high pressure was not needed. Caulk was pumped into first hole until it oozed out of the next hole and so on.

The flexible routing tool used to remove the balsa core was made from a Vermont flexible screwdriver attachment shown below. We used a Dremmel to cut blades into the end of the tool that normally is attached to the screwdriver bit. The Cutting head was forced into the core via the through hole opening. As much as a 7" radius of rotted core could be removed in this way. It is a very slow process however.

The cure for a rotted core is the same as a core with moisture except (1) much more polyurethane is injected into the core and (2) a stronger polyurethane sealant is used. Note, injection cannot be used to cure a core that is structurally unsound. If the core is spongy and stress cracks are abundant, the core needs to be torn out and replaced. But if you have a strong laminate that is a little bouncy then using injection to replace the balsa with polyurethane is a far less destructive cure.

Note, the following cure is theoretical and we have not used it to large areas in a rotted balsa core. However, we have very successfully used this cure to fix a 12" exterior fracture under the waterline in a balsa cored hull.

Drill small holes in back of the core. After the core is dry, replace the balsa by injecting polyurethane sealant into the core. Because polyurethane sets up in moisture and takes weeks to months to cure inside the core, the sealant works its way into voids and forms very good bonds. You will be surprised at how solid your deck feels once the urethane cures.

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

Use a moisture meter to find moisture problems in the balsa core. Map out the moisture variations into Moist, High-Moist and Wet. From the inside, drill 1/8” holes in a 1”x 1” grid and allow the laminate to dry out. This laminate was High-Moist when first tested. After going through 1 Minnesota winter, it tests Dry on the meter. 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 is used to drill holes in a 1” x 1” grid.


Stan Sroga



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