I had the same problem with my Danish Fish Cutter Gina 2 from Billing.
See blog here-
I sealed the balsa stern block with Deluxe Materials Eze-Kote which is a water based resin.
Two coats diluted with about 20% water so it soaked into the balsa, then two or three coats pure to add strength and scratch and knock resistance.
There are two Deluxe Materials importer / distributors in OZ,
one should be able to tell where you can get some🤞
Cheers, Doug 😎