I never looked and should have before posting this but does OGR even have a weathering forum? I created on facebook for folks to post photos and discuss their techniques and materials used and such, hell I can talk about this for hours I guess because I enjoy doing it lol. Some advice I can offer from what I found is and others may agree or not but everyone will eventually find what works best for them is to start off with dull coating the piece. Then I use a combination of airbrushing with acrylic paints, dry powders, and real dirt and rust. The reason I dull coat first is that if I feel something looked a bit too much or a whoops is that I generally can wipe it off with some warm water. Then when finishing I hit it will dull coat again, but this time from a distance as to settle on and not run off your powders and such, mist it in the air and let it settle onto the train. This is one of my earlier pieces and still my favorite, you know what they say about your first lol. Always will have a special place...
Oh and always protect windows and lights from being hit with the dull coat, tape them off.
