I’ve got some sleepers working as a low retaining wall, they are about a year old. I say sleepers but they are not actual sleepers, just sleeper sized hunks of timber. One of them has a knot on the top side, about 5cm diameter. It is holding water and is a mulchy mess inside. How do I dry it out and stop it rotting the timber? I was thinking about drilling out the mulchy mess until I find decent dry wood then filling with some sort or resin but I don’t actually have a clue about these things.
Ta

You can buy a product called wood hardener, but I’d get rid of any really mushy wood before I used it. Your idea sounds like a good one to me, maybe with the wood hardener before the resin. Might be overkill, but better that than having it rot again in a couple of years.

Whatever filler you use make it a little proud so you don’t encourage a pool to form in future.

Drill/cut out the rot, and shove a plug shaped to fit with some wood glue. Cut and shave flush afterwards.
It’s similar to what we do when moving hinges on gate posts to stop rot getting in and maintaining the structure.