couple cents to add: most *cleaner* automobile waxes are called that because they have abrasives that "clean away" old oxidized paint and dirt. If you are concerned about what an abrasive will do to fragile old decals, you need to go *very gentle* with a cleaner wax, or find one of the few waxes that has NO cleaner feature: Mothers make one but they also make cleaner waxes so you must read the labels carefully.
As Mswantek says, usually acrylic lac, such as used for artwork, is not as aggressive as nail polish (which is really just lacquer and thinned with lac thinner or straight acetone), but you have to read those labels to be sure. Since you know the solvent in nail polish attacks your decals, you want something that does not contain acetone. I've had pretty good luck with Krylon Crystal Clear which is a spray acrylic and can be misted on with very light coats and seems to be gentle on *most*, but not all, decals. The trick with it is to use a very light coat at first and allow that, and each successive coat, to dry completely before adding more: you want the solvent to evaporate away rather than heaping solvent upon solvent.