Do you still do the gloss-coat, decals, gloss-coat trick? It seems to me that if you clearcoat the metallized finish, you will reduce it’s metallic apperance. But you have to do something to seal those decals to the model. Confused and stumped [%-)]
You are correct in that anytime you add a translucent layer over a NMF you will change it’s appearance if even only slightly. For decals I will tone them down with my decal sealer which has a matt finish. You need to look at reference photos of the aircrafts markings. In the early days many of the markings were glossy when new, then they faded with exposure to sunlight. I drybrush the sealer to the decal only. This protects the decal, tones it down and doesn’t effect the NMF finish.
I have never “sealed” decals on metalized finishes…it is simply not necessary. Here are some examples of Alclad II sprayed directly over cured future. Decals were applied with normal setting solution.
Some folks like to use the gloss black primer method before Alclad II application. I tried it, don’t like it. I polish the bare plastic with any one of the plastic polish products out there (I prefere Tamiya extra fine) and buff to a sheen. Then apply future, let cure, and a flawless finish every time. Decals are a breeze.