The Natural Philosopher wrote:

Dunno if you can still get it, but there used to be a resin stabiliser
that soaked into rotten wood and turned it into a completely impervious
composite.

The best solution for that use is slow-cure epoxy resin. It's used by
boat builders to saturate wood with resin when making wooden boats. It
has low viscosity so that it penetrates wood fibres and when correctly
mixed it doesn't go off for at least 24 hours giving it plenty of time
to soak in.