Cat scratching post (cottonwood) -- sanded top smooth to remove chainsaw marks -- what to seal with?

If you are gonna let your cat mess with it, I would go with a food safe style finish you would use on cutting boards or the like. Might have to reapply occasionally, but better than running the risk of the cat chipping off and ingesting a more permanent type of finish.

This is an experiment -- if she prefers this over the couch, we may strategically locate a few before re-covering the couch. But ideally they double as end tables, so the top should be "finished" somehow.

If you could also top it with glass, it would be less likely that the cat could eat a part of the finish. Also it might give you more options as you would not need to be as concerned about water.

However I'd say leave it unfinished for now just to see if the cat actually uses it.
- Based on the bark debris around it this morning, she is definitely using it immediately.
  
  Glass has a side effect of requiring the top to be actually flat. I used the rotary sander to chew down enough of the surface to get rid of the chainsaw marks (60 grit, then 220), but it certainly isn't level or flat.
  
  Good suggestion though. If we end up doing a version two. Probably could go with a narrower log and centre mount a wider flat glass surface. Make it removable so we could replace the log after the cat shreds it up too much...
Unsolicited advice: there's wide, double-sided tape on Amazon that's inexpensive. It comes off without taking the couch fabric with it, and if you put it where she scratches, she won't. Giving her something else to scratch is a great idea, and it will help her change her habits if you also discourage her old scratching places.

Something like this - I'm not vouching for this brand, it was just the first result.
- This worked wonders for me 5 years ago, and still not scratching up the furniture. If you're feeling extra cheap like I was, you can double over (roll into a loop) packing tape so it's sticky on both sides.
  
  For finishing the wood: walrus oil (for cutting boards)

A+ cat impression

My experience with cats: unless you're willing to reapply the finish every couple of months as the cat removes it (in which case use a food-safe oil and keep in mind that cats notice smells) leave it unfinished. The cat won't care, and we've had several that actually preferred raw wood (the current one is a cardboard junkie, though, so he's happy with cheap scratching pads from the store).

Linseed oil