In Order...
Paint
Gloss-so the deacal has a smooth surface
Decals
Gloss-to seal decal and "fill in" around the edges makingit less likely any weathering will build up and make an outline around the decal.
Weathering- if it will be weathered not is the time to do it.
Seal coat- Flat, gloss, semi-gloss... whatever your final product needs.