I've been using Testor's Dullcoat spray (item 1960) since the early 90s, and still possess plenty of figs that I've used it on. No discoloration, and even the ones that I've used in games frequently show no wear or degradation. Of course, that could change next week. LOL. I usually give a finished fig 2-3 coats (letting it dry completely between coats) and I don't overdo each coat. It -can- cloud up your work if it's super hot/humid, something that makes a brush-on varnish attractive. But on days like that, I just wait a day or two until the weather changes and do all the spraying I need.
I've never considered 'long term' varnishing. The main thing for my tastes is that I -hate- 'glossy' looks, and demand a matte finish on my figs. There are exceptions, of course (slick-looking tentacles on some of my 'nids, alien mouths/jaws/tongues, etc), but in general, I avoid the gloss. It's not 'better', it's just a personal preference. I have on occasion hit a finished fig with a coat of the thicker gloss varnish, let it dry, then 1-2 coats of matte finish to tone down the shine. Some people claim that 'greatly increases' the protection factor of the paint, but I haven't dropped a bunch of figs to test that out! :)