"Spar varnish" used to refer to exterior varnishes specifically formulated for marine applications. They were typically long-oil varnishes (higher ratio of oil to solvent and resin as compared to interior varnishes). The higher oil ratio added flexibility to the finish so that it would not crack when applied to marine woodwork that was subject to lots of expansion and contraction because of exposure to temperature and moisture extremes. As chemistry improved, spar varnishes were formulated with UV protectants to minimize wood degradation due to sunlight exposure. Since case colors do not seem to fade from UV exposure, and since the metal in your shotgun does not expand and contract like a piece of wood, spar varnish is not necessarily the best way to protect case colors. The flexibility of spar varnish also makes it somewhat soft, and it typically (on wood) needs to be refreshed fairly frequently. Modern "spar" varnishes are often mislabeled, since they are often polyurethanes rather than traditional spar varnishes. The best traditional spar varnishes used to include McCloskey's, but the current formulation is junk (too eco-friendly to be useful for anything). MinWax Spar is a polyurethane, and is actually pretty good in some wood applications. Most big paint manufacturers will have a product that they call "spar varnish" but you'll need to read the label to determine what it actually is. Or, go with a high end marine product like Epiphanes. I'd stick with Dr. Gaddy's recommendations.
