I just discovered this stuff called "Tamiya Weathering Master". It is like a ladies eye shadow, it comes in these little cases and you rub this sponge into it and then apply it to the model....Here is a sample of it.....
These are nozzles from a F/A-18E I am currently working on. The one on the left is untouched....just painted with "Steel". The one on the right has been painted in Steel, and has also been covered in "Soot" from the Weathering Master set. This took about 5 minutes to achieve this. The nice thing is, if you don't like the way it looks, it is a snap to take it off and retry. Then, when you have achieved the look you want simply lock it in with some dullcoat.