Were it spar varnish (as we know it today), alcohol won't touch it unless you give it a good soaking and let it sit. Goodness knows what formulations were used in factory gun varnishes of a hundred years ago. The MS's I've owned were oil finished and appeared to my (then) untrained eyes to be original, and responded to gentle cleaning techniques such as scrubbing with linseed oil on a rag and then waxing. I shouldn't wonder that no single type of finish was employed for all MS's during the period pre-WWI.
