This is probably a really newb question but here goes, Every time I finish a model I spray it with a dull coat to flatten the paint sheen and help seal the decals in place. Even after spraying, though, the decals always tend to be shinier than the surrounding paint. It’s most noticeable with the Testors decal paper I use but even kit decals do this.
I’ve never had that problem, though I’ve seen it. I use the tried and true method of paint, Future gloss coat, decal, Future gloss coat, and follow with Future Flat coat (Future mixed with Tamiya Flat Base). I believe it’s the second gloss coat that seals the decals while evening out the surface so that the flat goes on smoothly.
I’ve never used Future before, maybe I’ll test that out, I’ve still got some extra decals on my Testors sheet, as well as left overs from the two kits I just used.
It doesn’t have to be Future, though it’s cheap and great for dipping clear parts in. It’s the gloss coat before and after the decals that does the trick.