You don't need no Tacky Glue garbage. As a rule that 99% of model builders knows that decals stick best on gloss coat coat surfaces. Besides, it looks tacky looking with glossy decals on a flat coat surface. After you're done with the decal session, spray a coat of flat clearcoat over the model kit to seal in the decals.
Honestly, it's laziness one would apply decals on flat surfaces when a simple quick gloss clearcoat is best. I am well aware most aircraft colors calls out for flat coat colors, but everybody knows you can go back and spray to a flat coat surface after decal session on gloss clearcoat. What's so hard about that?