Decals are best applied to a gloss coat, this helps eliminate silvering. And yes, once on, they can be sealed with a flat coat. If its only a couple of small decals, you can just gloss that spot. But if there's lots, it might be best to do the whole thing. I use a gloss paint which deals with that problem and saves me adding an extra coat.
One way I had it explained years ago was to think of gloss as a nice piece of flat ground. The decals will adhere to all of it. But Flat is like a series of hills. The decal sits on the peaks with gaps in between. The light reflects back off these low points which creates silvering.
I am a Norfolk man and i glory in being so
On the bench: Airfix 1/72nd Harrier GR.3/Fujimi 1/72nd Ju 87D-3