The way we preserve carvings
I put boiled linseed oil and paint thinner mixed 50/50 on the trunk. Give it as many coats as it takes until the mixture no longer soaks into the trunk. This may take several coats. After you get to the point that the mixture no longer soaks in, let the trunk dry for a day or 2. Then coat the trunk with spar varnish, or marine varnish, mixed with an insectacide, you can find spar varnish at any hardware store and mix it with the insectacide. Put at least 3 coats on, or until you reach the desired gloss.
Keep in mind that the trunk will still continue to draw in moisture thru the roots, so there is the problem of rotting,insects,ect. This is the best way of protecting a trunk that I am aware of. Carvings that we made in trunks 8 years ago are still doing fine. We tell our customers to clean and put a coat of spar varnish on once a year, but that there is still the posibility of damage. I hope this information is helpful.
