Gloss-Decal-Matt

Bopth answer are correct no douibt about it. Here's a way to visualize the "why" decals need to go on a gloss surface. Picture that you have a very thin sheet of clear plastic. Now lay that down of a sheet of 100 grit sandpaper. The sheet of plastic will sit on all the high spots of that very rough surface with a lot of air space underneath. If you looked at flat or matte paint under a microscope the surface looks like sandpaper. The clear parts of the decal will look "silvered" from the air space under the decal. Now lay the sheet of clear plastic on a sheet of glass. 2 smooth surfaces facing each other. No air space... no silvering.

This is a pretty specific question for a general modeling discussion forum.  For a detailed discussion I suggest asking in the decals forum.  But short answer is, do not apply decals to a matt surface.  Either gloss coat the whole aircraft, or apply a gloss coating to areas where decals go.  Then, after decals are applied and set awhile, apply a dull coat.

sherbir - please excuse me saying that, but without decals that Typhoon is far from completed!

As for your questions - applying decals to a matt surface is risky, at least. You would need lots of luck and good decal softener/solvent to apply decals like that without silvering. So yes, gloss coating your model before applying decals is definitely a good idea. And you still need to watch out for silvering. If it occurs, consider using mentioned softeners/solvents, or adding some diluted clear varnish (future?) under the decals. And please be careful here. Once the decals are dry, it's a good idea to put another gloss coat on top of them, to get a uniform surface, and then put the matt varnish on top of that, as the last coat. I hope it helps, good luck with your decals and have a nice day

Paweł