Abastyr: I am using the same technique as everybody else except that I don't apply a clear gloss coat on the model before weathering.
Note that a clear gloss coat is very nice to seal a painted model and will allow good setting of decals. Also it will allow the wash to easily run down the sides of the model.
IMHO however, for dirty water to run completely along the sides of the real vehicle the flow should be much or the surface is very smooth. I work for a construction company and I see a lot of heavy equipment. Even on a new equipment once it is used, a thin layer of dust accumulates on its surfaces. A light rainshower will cause dirt spots on the surface and the dirt will not fully be washed away. Some scale like marks will even appear once the vehicle is dry again. Only when really heavy rainshowers occur that all dirt will be washed away leaving only streaks of dirt in some parts.
I think I can simulate this by not using a clear gloss. It is harder to get good decal bond but I can always get the uneven dirt/water marks and streaks that I want to simulate because the wash will not freely run down the sides.
Also when I apply a wash, I seldom remove the excess. I just shake/tilt the model to allow the wash to scatter or flow to the recessed portions.
I use a brush to paint the streaks or dirt spots wherever I want.
I also drybrush but lightly. Drybrushing heavily IMHO could give too much depth that will make the model look unrealistic.
But these are just my opinion and I am not an expert either. A lot of guys could say they are doing it the other way and they always come up with very superb works.
LQ2: Giving a rusty look to a model is just using lots of red brown and flat brown. You'll get to it I am sure. Afterall you did a great job on your Panther G. When I saw your camo, I decided to repaint my old Panther G. I actually used the wrong Dark Green that's why it turned out too dark for the scale.
To all: Thanks for the kind works. This means that my work is improving. Thanks for the tips I get from you guys.