Setting solutions are the two-bottle system that makes waterslide decals look painted on. The setter goes on the surface before the decal to improve adhesion; the softener goes on top after placement, gently melting the film so it drapes into panel lines and around rivets.
The cardinal rule: after softener, do not touch. The decal wrinkles alarmingly, then pulls taut into every recess as it dries. Poking mid-cure is how markings die.
See it in context in our panel lining & decals guides, or explore the tools hands-on in The Bench.