Thanks for the feedback guys. I'll sort out those bugs and have another look at the weathering, maybe tone down the filters and add some more specific oil and dirt stains etc.
The paper bag filter Cit refers to is done this way:
Firstly one of my paint layers always covers the whole of the painted area of the airframe. This is used to select the areas just needed for weathering effects, like the overall dirt filters.
Having selected these areas I use an airbrush tool set to very low density (1%) and airbrush over the whole area in some shade of brown or grey. I start with a very wide tool then go over it again with a narrower settings while upping the density slightly, concentrating on dirtier areas, wing walkways etc.
A low density airbrush looks too grainy by itself so I like to blur it with either a 1 pixel gaussian blur or a 5-6 pixel motion blur. The latter is set to the direction of the airflow, sometimes you need to select different areas of the filter and blur them in different directions.
I usually create four of these overall dirt layers using different shades or brown or grey and different blurs. When all the filters are faded right down they should merge into a random looking dirty effect. Note the above filters are just to "age" the paintwork, they shouldn't be noticable unless you are looking for them. Things like exhaust and gun streaks, dirt stains and oil drips have their own layers.
