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)

What I do for weathering :
- create a new layer; I name them "dirt map_01" etc, as many as you need.
- fill it with a texture that will create contrasts between part. I use sometimes a metal texture, or it can be a paper texture, etc (shipped with PSP or found on the net).
I choose them according to the pattern it gives.
- if the texture is in colour, desaturate it completely.
- play with the blending modes and the light and contrast in order to get the variations on the paint without altering too much the base colour.

Then there are all the little stains, scratches you can add later around some panels. But that is another story...