I've used all these: Badger's decal preservative, Testor's Decal Bonder, various spray can clear lacquers. All work. Badgers caused the decals to curl up and took several applications of Solvaset to lay down. Clear lacquers also somewhat stiffen the decal and present a little problem laying down with Solvaset. Testor's Decal Bonder has never failed to give excellent results even when the decal could be seen cracked on the backing paper.
I have the MicroScale product and intent to give it a try, soon.
I'd also like to try an acrylic clear. Possibly on an extra decal set. Possibly Createx's two part clears.