This is my idea for weathering or designing the grime effect on burned (not real burned ;) ) Lego models with makeup powder. here you can see what i use for it and what results are after using it. Hope you enjoy it. Feel free to try it out by yourself ;)
About this creation
This is the makeup what i used. If you want to clean the Lego's, use warm water with soap.
First I would recommend using chalk for this job. Much easier to handle and easier to cleanup if you need the bricks for another project. You can do it in every colour you want. It's so funny watching the old blurry pics. Improved my photography a lot over the years, LOL. Take a look at this one from my friend Stephan. All weathering is done with chalk, the bright spots at the tires too. I think it's the much better lookin solution. https://www.flickr.com/photos/stephann001/sets/72157694153391405
Oh man... such shame that I missed this nice comment such a looooong time... After so many years I think you found a solution or tried it out. I'm so sorry... Btw, things changed a bit and due to a friends outstanding work I found out that chalk is a much better solution.