Question on some finish work for my GMC 6X6...

Try masking the area where the windshield wipers swath would be, then LIGHTLY dust on some light tan, or other light earth tones with your airbrush. Passes with the brush should be quick, and at a little distance. FYI, Folquil makes some excellent weathering colors (dust, mud, grease, etc). I have used them with varying degrees of success.
One other thing that has worked well for me is to take a compass (dividers are better since they have 2 points), and put one point into the base of the wiper arm, and extend the other point to the outer-most wiper edge. Then, using the point on the compass, scratch the arc into the windshield. This makes excellent scratched windshield glass. Couple this with a mask and dust (as above), you should get a good effect.
Gip Winecoff

Thanks....but I unfortunatey don't use an airbrush. I am assuming this won't work all too well with just a regular brush??

Not really; however, there is an alternative. Thin some of the color you want to go on the windshield, then load a toothbrush (preferably and old one!) with the paint. Now pull the bristles back with your finger and let go. Great spatter/splatter finish. You might even try it with unthinned paint. Experiment on some old stuff before zinging your pride and joy!
Gip Winecoff

Thanks styrene!

I looked back through for the article on muddying armor and found it-- your spatter method reminded me of it-- much appreciated! I'll have to think on this windshield-- to be honest-- I am wondering if mud would've made it all the way up to the GMC's windshield....

I am soon to start Tamiya's Willys Jeep and that will definitely have some mud on the glass!

The windshields just got plain dirty! Road/off-road dust settled all over convoys...and then it rains. What do you get? MUD. Not great globs of the stuff, but certainly enough to obscure driver vision to have to run the wipers to remove it... Keep us posted on your progress, and post pictures if possible!
Gip Winecoff

OK...I owe you guys (especially shermanfreak and guntruck from armorama)...so here are a few pics:



Not bad for a first-time vehicle modeler who's used to planes, eh? ;) I know-- there is some weathering to do LOL

And for my first time playing with celluclay-- this needs a couple days drying.... I just hope my 1:35 scale RedBall Express sign gets here soon LOL....I'll add some rusty debris as well....



I'll put up finished pics when it is.....

That's the stuff we want to see Jeeves ..... pics, pics and more pics.

You're off to a very good start with this one ....... get some mud on that baby and watch her come to life.

Thanks for sharing.