I am building Tamiya's 1/48 Protze.What is the best way to depict a dirty or dusty windshield,perhaps even the wpier marks ?

Maybe if you ask this guy he has done a nice job of it.
Example

Anthony,
Mask the windows in the shape of the wiper marks with Blue or masking tape. I cut the masks using an Olfa circle cutter and them trim to the right arc length with an x-acto. Apply the masks to the window prior to final weathering. I will overspray the model with a real thin light tan color such as Tamiya Buff. If you want an even more subtle effect you can spray the windshield with a light coating of dullcoat and it will just "frost" the glass.

I have on occasion, masked and weathered the glass off of the model and then installed it at final assembly.

HTH,
Dave S.

sounds good.............thanks

Another easy way to do it is with a circle drawing tool. You know the oneyou can get in any craft store made out of clear plastic and has a lot of differant size holes in it? Measure the width of the wiper BLADE. Not the whole arm. Then multiply that # by 2, and you will know which size circle to use on your tool. Next, draw the correct size hole on some frisket paper. Frisket paper is nice because you can re-use it after your done. next, cut out the circle on your frisket paper with a scissor or circle cutter carefully. Then, cut the circle in half. Lastly, cut out a half of a smaller circle on the top center of your half circle, and your good to go. Apply it to your windshield where the wipers are, and either dust, drybrush, or use pigments to dirty the remaining clear areas of the windshield. Unfortunately, I dont have any pictures of the steps to do this method, but generally, it comes out looking like this.....................

That picture is missing dust inside the wiper arm's reach, as if the wiper got longer on the inside edge mid arc.....

Yeah, I know. I corrected that recently. Unfortunately, its the only photo I have with clean wiper marks. If you read the instructions I posted, you'll see I mentioned to cut out a small half circle at the top of the frisket paper.

C'mon Kevin.... Gary has the gist of it and showed it well. Okay. So he's updated it realizing there was a "mod" needed. End result is a good depiction of what Anthony is looking for.

Mike

jeez it was just advice, I'm not Kreskin and knew he had already corrected it....

C'mon Kevin, you were supposed to know. Didnt you pass your ESP class??? :-) Its all good. You were right. I was going to do a review on the barriers in that picture some time ago, and just never got around to it. The humvee and figures were just in that picture for a scale referance. I still had more work to do on the Humvee when I took that picture, and I really never bothered to update the picture as the Humvee is in a diorama at the moment. Have a great weekend guys!