I'm far from an expert, so I think its a good topic for me too. I use more dilute paint. Polyscale about 3 or 4 to 1 with rubbing alcohol. Since its so dilute I use a slightly lower air pressure setting for my two stage air brush, around 25 psi. I spray on a light amount, with the idea that its easier to add than to remove. Usually takes several passes to get the effect I want. I have not done much recently, but I know as I do more I get better and it goes quicker. Also found that just a little weathering is often enough. Especially if I'm just trying to make something like a freight car not look too new.
Mark
For the past few years, I have been doing most of my weathering for my structures with dry-brushing and chalks. I finally got my airbrush and compressor out of storage. I hope it works after 5 years! It is time to get it working.
What is your favorite technique (paints, colors, mix, settings) for using the airbrush to weather structures and equipment? - Nevin