Repairing a rotten (balsa)core

Many polyester sailboats have a balsa core deck covered with polyester and fibreglass. Speedboats often have a structure of trusses, made of sheet material covered with polyester. Due to the penetration of moisture (for example through holes made in the deck to install fittings), cracking in the trusses or wood that is not 100% covered, the wood can become soft and can 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 spots

Supplies:

• Epoxy + filler
• Syringe
• Topcoat or filler
• Application materials

Small spots rotten balsa core are fairly easy to repair by the following steps:

1. Drill several holes through the first layer of the balsa core.
2. Smash the affected wood with, for example, a piece of bent steel or an Allen key.
3. Vaccum it out and allow the surrounding wood to dry thoroughly.
4. Fill the whole section with thickened epoxy (possibly with a syringe).
5. Finish the holes and pay attention to the cause of the rot. Seal this well too.
6. Finally, you can mount the fittings in the same way, by drilling through the epoxy again.

Also view our article on gelcoat repair and finishing.

Larger spots

Supplies:

• Epoxy + filler
• Core material
• Glass fabric/glass tape
• Topcoat or filler
• Application materials

You can repair large spots rotten balsa core by the following steps:

1. Grind open the spot where the rotten wood is. It may be a lot bigger than you expect. Make sure you remove all the rotten wood because if any rotten wood is left behind, rotting will continue. For grinding, use a grinding wheel on an angle grinder. When doing so, make sure you only grind through the top layer.
2. Remove the rotten core, this can be done with a chisel, for example. 
3. Now insert a new piece of core material. This can be balsa or plywood or hard foam.
4. Glue back the (cleaned and sanded) plate that you sawed out earlier and laminate the edges with some glass tape. You can also choose to re-laminate the entire piece. You can read all about this in the articles Repairing polyester and Finishing gelcoat.