MS, check out Git-Rot. I used it as a kid when my parents had an older boat with a wooden superstructure and then again on an old plywood runabout that I had shortly after. While my father wouldn't tolerate much soft wood, he would use it if he discovered any mid-season and it really did harden up stronger than the surrounding wood. On my runabout, well, it probably had more Git-Rot than solid wood. Realistically (as opposed to ideally), if the wood has rotten to the point it is crumbling apart rather than just being soft is should be replaced. Ideally it
should be replaced but...
That Git-Rot really seemed to do the job.