The chalk tip is something I have not used on tanks, yet. I use both water based and thinner based paints. The acrylic paint you mention is something I'll have to try, given the texture it can achieve. One thing I have tried is mixing both an airbrush effect with flicking paint from the bristles of an old tooth brush. Gives more randomness to spatters and stuff.

Another way to simulate heavy caked mud is by using some stuff called Liquid Steel! Natural mud is better, but I wanted to be able to clean it and still have a muddy tank. I was trying to fill the holes in the body where the mud guards bolt on and found the LS filler had the same texture as mud. Paint it up with some dark earth colored paint for texture highlights. Since the filler itself is a very dark grayish color (almost black) washes of the paint made it look even more real.

Since control is still an issue with my airbrush (operator error), I use a brush for all most all of it. In the future using the airbrush for at least the base coat, as well as parts of the weathering will be in order.

In those new pictures you can see the weathering. Looks good!