The milk treatment worked really well for me (I gave details in that thread). To a certain extent, anyone who drinks hot coffee with milk from their kuksa treats their vessel by so doing (which would help to explain why some people find that their kuksa stops leaking after a few coffees), but all I did is boil some milk and pour it in and leave it to cool. I'm no scientist, but so far as I can see from a little cursory reading on Google, casein is hydrophobic and resistant to dissolution by alcohol.
I finished my kuksa with walnut oil (from the outside, though obviously it soaks through) first, mainly because I like the finish. It's fine with both hot and cold, alcoholic and non. I was going to use beeswax but didn't want to buy any right now, and I've read reports of it melting under hot liquids (which would make sense).
I've read that pine resin makes a good finish, and you can of course use varnish (which can be natural). To a degree I think if your vessel is going to crack from the heat, there's only so much you can do to stop it.
