I'm big on sealing with a clear coat, but -- especially with the SS X-Wing -- you'll get varying responses. The original finishes were accomplished with a steel wool rub down which offers a unique (and familiar!) sheen. But I personally use a lot of pastels, so pretty much have to seal my weathering. I use Testor's Flat (or Dullcoat) from a rattle can, which goes on super thin and virtually NEVER orange peels (unless its way cold and goes on way heavy). I then buffed the clear coat with steel wool.
Chips and scratches... Lots of aproaches, and mostlikely very similar to the old war birds you work on, Fred. Some here use latex masking, others use the so-called salt technique. Personally, I just brush on the white paint over the top of the main color (like the red stripes). It's easier, gives you much more creative control over the shape of the blemishes (especially if you're stringently following ref pics), and if done properly, you can't tell its painted over the color (opposed to chipped away) unless you have a magnifying glass or are otherwise TOO close to the model.
For applying dirt and grime, I use pastels a LOT. Applied properly, you get the same look as an airbrush, but again have better control (well - I SUCK at airbrushing!). Just make sure the colors are natural. I have a hard time getting natural browns; most of my X-Wing was done with black pastel powder. I do like to thin dirty looking oil-based colors with thinner because I like the way it reacts as the thinner breaks down the paint on the surface of the model, but this is a dangerous technique as the thinner evaporates quickly and can leave a muddy mess, and too much of it can begin to attack the undercoat.
--Rob
