Need some advice......

Hi Andy,

Don't count on me being an expert, but at least did a muddied rally car a while ago, which was quite fun and might be applicable. 

For both the undercarraige and the windshield I used airbrush first , followed by flicking a toothbrush's brisles (to create some more "rough" splettering for a better effect). And I simply mixed some Tamiya acrylics, brown, yellow and I think a bit of white till happy with the colour.

And that's about it... Enjoy yourself and don't be afraid to expiriment. And remember to do some testing first!  

Best of luck - keep us posted. 

Now, over to the real experts... 

You can dirty up a window by spraying it with dull coat.  If you want to show wiper tracks tape that area before spraying.

I've herd of mixing a wash or a paint with a little fine dry dirt, or just sprinkle a little on with a salt shaker for a dusting. I havent used this but have herd of it on dioroma forums and books, and have seen good results. if you try it tell me how it looks!

I use a product by Tamiya called Weathering Master:

http://www.tamiyausa.com/product/item.php?product-id=87079

It's VERY easy to use. Just paint your flat black like usual and when it's dry apply some "dirt". There are also other sets with things like rust and snow but I get the most use out of Set A. After applying it you can spray with a clear flat to make it more durable but that's not necessary unless you expect considerable handling.

Don't count me as an expert either, but this is what I did to my Impala. I wanted the car to look like a well kept but used enthusiast car.

What I did was to mix some dirty thinner with Humbrol's buff and sprayed it on holding the airbrush pointing rearward at a low angle. Put some more on around and behind the wheels and also gave the engine compartment a treatment at the same time.

I don't know how to make more heavy dirt like mudd and such?

 

Good luck!

Niclas