If the loco was kept in covered storage except during use, you might be able to get away with a furniture type finish, although cinders and such would give it a hard time. But that kind of finish would not last very long in the great outdoors, so you would have to be continuously refinishing or watch it degrade and give no protection to the underlying wood structure, which would then begin to degrade. At that, the UV light at 8000' will cause damage to the wood with only a clear coat. Even as it is, it will require more painting then a steel cab would.