Here's my formula... First flood the whole model with a dark wash, either a very dirty brown, such as sepia, or maybe a much darker shade of your base coat. I personaly use water-based artist colors at thisstage, mixed with water, a drop of dishwashing liquid a a drop or two of vinegar. I will not encourtage you using enamel washes over enamel base paint, because you might simply liquefy the base coat... Acrylics are better but hard to remove if you've overdone it.... If you use enamel based wash overLet dry. Next, using a soft, flat brush, and a slightly lighter shade of the base coat (very little paint, and before applying, remove all excess into a cloth, so that the brush is just about dry), gently dry-brush over the whole tank, but mostly over straight edges. The little paint you have on the brush will slowly accummulate of those sharp edges and anything sticking out, givng you more depth than the wash alone could do.
Repeat with lighter and lighter colors, being even more gently as before.
Rust marks and grime can then be added, for instance using some small flat brushes and artist oil paints (black, sepia and Burnt Amber). Put a bit of paint in the spot you want, then use the flat brush to drag the paint in the direction (normaly down) you want.
If your base paint is enamel, and that you want to weather your model with enamels, make sure the base paint has dried for a few days before drybrushing. If you use acrylic for drybrushing, or acrylic over acrylic, you won't have to wait that long.
Finaly, use pastels and a fan-type of soft brush to add the coat of 'dust' all over the model, but mostly on the chassis/wheels and bottom of the hull.