Hey Stiz,
Create a new Layer and call it Weathering or something. Using the Airbrush Tool, draw where you want the weathering. Now go into the Effects tab and look for Blur. From there go into either Gaussian or Motion Blur. Gaussian Blur will just blur the paint on the layer. Motion Blur allows you to blur the paint in any direction (Perfect for streaks around engines and on the floor in VC's)
Experiment with them until you get the right result (Aso use the Layer Palette to very the Opacity)

Regards,
Stratobat
Edit: You can also fade the paint on the upper surfaces (Upper Fusleage, Wings and Elevators)
