Well, you can put the decals directly on the paint. Nothing to stop you from doing that. But unless you've got magic decals, they'll silver up and look terrible.
Your other option is to put a gloss clear coat over your paint. Future floor polish has a lot of fans. Testors gloss, heck, even Krylon clear gloss will work. Once you have a nice, smooth, gloss surface, apply your decals. Then seal them with another clear coat.
On the Bench: 1/32 Trumpeter P-47 | 1/32 Hasegawa Bf 109G | 1/144 Eduard MiG-21MF x2
On Deck: 1/350 HMS Dreadnought
Blog/Completed Builds: doogsmodels.com