Oboe -
Here is a pic with 1 layer weathering only, it is the fore/aft dirt streaks. I usually do up to 3, 1 long thin stripes, 1 shorter fatter (but less prominent, and 1 blotches (faded even more). I have accentuated the 1st layer to about 40% opacity in the pic so you can see it.
Before
1st layer only
Add a little more plus details
For wing dirt I use 90 long 10 wide, fuselage, tail etc 90 long 5 wide.
Not sure if the old eye candy is still available, I have a copy if you need it. New one is preview only and half the filters are disabled.
Tutorial for it is on skinners Heaven.
Basically create new layer, flood fill with white.
Open plugins and select eye-candy, HSB noise filter.
Set parameters according to Skinners Heaven.
Apply the filter and set the layer property to Multiply, play with the opacity. Takes 2 minutes
By changing the random seed value in HSB noise you get a different pattern each time.
Eye candy is a 2mb file if you have an ftp site or if your email will accept files that big.