it is also a matter of preference, but always apply decals to a clear gloss layer. flat clears don't help prevent silvering. a gloss coat wont guarantee no silvering, but it reduces it significantly and it may not happen at all. Also I prefer to spray clear gloss to seal the decals down, and then spray dullcoat. The dullcoat solutions out there can sometimes eat into the top surface of the decals, so another clear gloss then dullcoat helps prevent that.
Also, if using the Metalizers from Testors, apply a coat of their sealer. then clear gloss. On my kits I sprayed the Metlizer, waited a day, sprayed the sealer, waited 2 days, sprayed Future, sprayed Future again the next day, and today I'll start applying decals. The future is supposed to be able to be walked on in 30 minutes, so I figure if you can walk on it in 30 minutes, you can decal in a day and be plenty safe. Also dont apply decal over decal. If you have a decal that goes over another decal, like a kit decal over an aztec decal, apply a coat of clear gloss, let it cure, then decal. Help prevent decal on decal violence.