I recently found out that airbrushes can be for more than just base coats. You can use the airbrush for parts of weathering your vehicles or armor that don't have a hard edge like the paint chipped or metal scratches as posted previously. With an airbrush you can achieve effects like burnt paint around shell or battle damage, flamer damage, or carbon build up around exhaust or boosters.
For this example I'll take my Wave Serpent I've recently been working on. I used Mr. Surfacer 1000 to prime the model and then I got my airbrush and some vallejo paints out to start shading and adding small amounts of weathering here and there.
I first used Iraqi sand in a very thinned down coat to add a bit of coloring to the surface of the whole wave serpent. This would now serve as the base for all the following layers. I then used Brown Violet to go in and add all the shadows into the recesses and corners of the model. I then used Deck Tan to hit the highest points of the model creating highlights. Now so far I've just done a basic shade and highlight which will add a lot of depth to the model that I won't have to go in and layer up with a brush later. To start the Weathering I take Vallejo's Smokey Ink and thin it down and start applying it where I feel burnt paint would be. The walls of the chassis right behind the boosters received a couple thin layers of this. The vents going into the front where air would rush in and bring any dirt or debris with it. Also the small stabilizer boosters on the bottom of the vehicle and anywhere else in general I thought there would be burnt paint or darkening of the surface. This is the steps I've gotten too on the pictures below. To finishing out the weathering with my airbrush after the whole model is painted I'll take some flat black and apply it to the edge of the boosters and the troop transport part that's located right next to the boosters. This flat black will represent the carbon build up from the boosters and go ontop of the burnt paint effect already applied to the area. I appologize for not having pics of the carbon build up yet.