As Pawel said, I don't believe in any pre-shading, panel outlining, etc. A real helicopter just doesn't have any of those artistic finishes, so why add them? I don't prime either, especially before joining the fuselage halves. That only means you have to scrape or sand the paint or primer from areas where they will be glued together. For weathering, I do it all after the color coat. Usually I add an overall wash of a dusty color, maybe pick out some chips and dings arount heavy use areas, and some darker grey/black washes around intakes and exhausts. That works for me.