I've learned to use silicon dielectric grease (aka "high temperature brake grease" and a few other names) on almost every electrical connection. It is the same thing sold as ignition grease, except that's about $4 per 1/2 ounce and this is $4 for eight ounces.(G) Unlike petroleum grease, it is non conductive and will not cause the extra battery drain that migrating Vaseline films will cause. The "varnish" and purple spray and all work, as seals. But the grease goes right on the contacts, and keeps out all moisture and oxidation.
If I'm planning on a real long-term connection, I'll be sparing with the grease, and make sure the exterior is all clean and bare. Then I'll coat the whole thing with about 6 coats of 'liquid lectric' to build some thickness to it. That stuff peels off easily, but bonds tightly. First time I did that some highly certified mechanic starts to rip it off saying "That's the problem, it has to be all corroded in there" and I just stared at him while he looked at the pristine metal he'd just exposed.
If I'm planning to open something up within the next couple of years, I'll generally skip the liquid lectric, it just takes too long to wait for each locating to dry.
Many ways to skin a cat. The only really important thing, is to skin it before cooking it.(G)