Exhausts, gunbarrels and breaks usually have a little or a lot of soot on them. The best way to recreate this on a model for as far as I know is by pastels you can buy in art and hobby shops. I myself use pastels from 'Van Gogh' of the hardest quality. The pastel can be applied by first scraping some of it off with a knife. This 'pasteldust' can then be made more fluid by a little bit of water and applied with a brush. As long as the pastel is wet it can be (re)moved easy once it is dry this is more difficult but it can always be dissolved in water again.
The reason to use pastel is that it forms a completely matt surface because the pastel consists out of small dusty parts just like real soot but without the property of sticking to everything. Once it is dry it sits almost as good as paint on the model!
Just one warning! When you mix the black pastel of "Van Gogh" with an other color it truns to green. I don't know how if pastels of other companies also have this property but it makes this one unsuitable for mixing.
Some examples of pastel applied as soot: