X-wing advice

Well, I've not yet built the FM X-Wing but I use the same approach in painting and weathering WWII aircraft.

Once I've put down the camouflage coat I will put down a coat of sealant, either a gloss clear coat or Future.  Following that will be decals (you need the smoothness that a glossy clear coat for the decals).  I'll put down some Microsol decal setting solution and let that stuff do its thing.  Then weathering in whatever form I choose for the build in question.  Typically this is a combination of ground pastels and airbrushing oily streaks.  If I'm not using any pastels then I just put down a coat of flat clear to seal everything up, but airbrushing anything will make the pastels go away so I'll put that on last.

You might find this helpfull aswell.

http://www.starshipmodeler.com/tech/aw_wx.htm

Good link.