Repairing a rotten (balsa)core
Many polyester sailboats have a balsa core deck covered with polyester and fibreglass. In speedboats, a structure of trusses is often found, made of sheet material covered with polyester. Penetration of moisture (for example through holes made in the deck to install fittings), cracked rafters or wood that is not 100% covered, can cause the wood to soften and even rot.
You can notice this because the polyester compresses easily and feels soggy. Or because water is leaking from the holes in the deck.
Small sections
Supplies:
Small sections of rotten balsa core are fairly easy to repair through to the following steps:
- Drill several holes through the first layer of the balsa core.
- Smash the affected wood with a piece of bent steel or an Allen key in the drill, for example.
- Suck it out and allow the surrounding wood to dry thoroughly.
- Fill the whole section with thickened epoxy (possibly with a syringe).
- Finish the holes, also paying attention to the cause of the rot. Seal this well too.
- Finally, you can mount the fittings in the same way, by drilling through the epoxy again.
Also check out the article on gelcoat repair and finishing.
Larger sections
Supplies:
You can repair large sections of rotten balsa core through the following steps:
- Grind open the skin where the rotten wood is. It may be a lot bigger than you expect. If any rotten wood remains, the wood will continue to rot, so make sure you remove all the rotten wood. For grinding, use a grinding wheel on an angle grinder. When doing so, make sure you only grind through the top layer.
- Cut into the polyester/fibreglass and remove the rotten core. This can be done with a chisel, for example.
- Now insert a new piece of core material. This again can be balsa or plywood or hard foam.
- Glue back the (cleaned and sanded) plate you sawed out earlier and laminate the edges with some glass tape. You can also choose to re-laminate the entire piece. Read more about this in the articles Repairing polyester and Finishing gelcoat.