A:  Thorough and professional repair of rotten balsa core involves replacing all affected material. Simple as it sounds, if one third of your balsa core is actually rotten, this procedure will involve a great deal of time and effort. There’s no quick, easy or cheap fix —be cautious of any suggestion otherwise.

Balsa core is a great material, but with some very unforgiving limitations in use. It must be installed correctly, and it must be isolated from any water intrusion or the problem you face is the inevitable result. Be sure to understand how the core got wet and plan very carefully how to prevent a repeat.

Balsa that is infected with rot (it will be darkly discoloured) must be completely removed. Wet core that is rot-free (still looks like tan-coloured wood) can remain, but it must be completely dry before the structure is closed up. A proper repair will entirely seal the repaired core, so any moisture left inside will remain—and likely feed further rot.

Once the deck is opened up, it could take weeks in good conditions to dry out the core, so it may be more economical to replace all wet and rotten core rather than wait for existing wet core to dry. A moisture meter (calibrated to a balsa-cored panel known to be dry) will provide a standard for comparing readings taken over the entire deck. Map out the wet areas and take core samples to determine the extent of rot. If the core has delaminated from the skins, the repair must extend (at least) to a properly bonded deck.

Removing large areas of deck skin will weaken the boat. Plan the repair to avoid distortion of the hull while the deck is severely compromised. Repair may have to progress in stages and/or the hull may need shoring to retain shape. Manufacturers’ recommendations for installing core material must be followed to the letter.

After the deck skins have been replaced, all structural repair laminations must be refinished. This aspect of the job may be the most time consuming, and requires a great deal of skill, especially if the level of finish on the boat and your expectations are high. Planning is of the utmost importance for good results with this phase as well. You and the person doing the work must be very clear about the quality of the finished work. Presumably you’re not going to all this expense without expecting the finished appearance to be as good as or better than it was at the start.

This will be serious and expensive work that will determine the future value and integrity of your boat. We recommend you seek the services of a reputable, experienced repair yard that employs qualified technicians. Enquire about similar jobs the yard has completed recently. Contact some of these people if possible for a customer’s-eye-view of the process.