Well, varnish on leather is one thing, and a sealer on enamel or acrylic paints are another, but in either case, it can't prohibit fading, only inhibit it.
I use Future floor polish to coat and seal my figures, when I paint them. Some have said that Future will discolor over time, too, but I don't think anyone's used it long enough to notice a big difference yet.
I have noticed that some older figures that were clear-coated, like King-White/Reeves figures, have taken on a yellowish tint with age. KW/Reeves is a good example to be able to compare coated and uncoated figures, because they made their figures in both finishes, so we can often compare the same figure, in either finish.
Prost!
Brad