For vintage GI Joes, there are a lot of things one can varnish with. As they are hard plastic, heck, you could even dip them in thin clear resin. The problem comes with VvV or 25th figures, as the rubber won't accept most of solvent based stuff.
I was the other day trying to scrub off enamel paint off a old ARAH body. It wouldn't come off for anything in the world, I ended up painting the same plastic original colour over! hence, just regular model kit enamel hobby paints, well dried, in a thin layer over a slightly sanded surface, will be nigh-indestructible.
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See all my customs in high resolution on my blog;
http://cobrafactor.blogspot.com/