i ddon't know if that is theyr name but i mean those things on the front window. how can i make sdust on a window and a part that is clean

screen wiper, i think

:-)

I think you are talking about window screen wipers. I think the best way to achieve the effect you are after is to mask of the area where the wiper passes and then spray the window screen to get the amount of dirt you require and then simply remove the mask leaving a clean area with a dirt effect around it.
Hope this is of help to you
Robbo

Masking is the key.
There are two ways (that I've used and seen posted) to mask the 'clean' area on the wind screen. You can cut a piece of masking tape in an arch that would match the path of the wiper. Tape it in place air brush over the wind screen in Tamiya Buff color or what ever color you are using for dust/dirt. Spray in thin light coats so you don't make the rest of the screen opaque. Remove the mask once the paint is dry.
The other mask agent is liquid mask. Simply apply the mask to the area that will be clean and spay as described above.

Vlady, here's one I did using the tape method that slodder described

hey guys i have a prob. i don't have an airbrush and i don't know how to mask. he;p please

Vlady,

Masking should be done like Slodder said.

You don't have a airbrush? Not a real problem, you can use pastel to create the dust effect

vlady,
In this case masking on clear plastic with no paint or undercoating of any kind you can use any kind of tape for a masking agent. You can use the cream colored 'masking tape' found here in the US. You could use transparent 'scotch' tape used for wrapping gifts. I know Tamiya produces a number of different kids of masking tapes.
RoadKill hit the nail on the head - no airbrush - no problem - Pastels will do.

pastels means what? i don't understand

vlady,
Pastels are artists chalk. They come in two kinds oil and chalk - you want the chalk kind.
They are sticks of colored chalk that come in a wide varity of colors. You use them for Many different things.
The basic technique is to use the back of a knife or file to scape the desired color into a small container. You will have a fine dust to work with. Using it is similar to dry brushing. When I use it to weather an AFV I take a stiff(ish) brush and work some pastel dust onto it then I 'dust' or work the color onto the AFV where I want the weathering. Nooks, cranies, running gear, fenders, etc, etc.
Use a thin coat, light color for dust...
Use a thick coat, dark color for dirt....
You can use a light dusting on your wind screen too.
Another use is for as a pigment to make a custom color. You can scape off multiple colors into the same container and then add water until you get the right consistancy and use it like paint.

Hope this helps.

i still don't get it but thz anyway and i will ask for someone who speaks better english then i do and maybe i will het it. thz anyway to all

I have used a flat bristle paint brush the length of the wiper blade and wiped off a thin wash getting the effect of a dirty winshield with out using an airbrush. Just like the real thing but in a smaller scale. good luck.