the method that I find most realistic is by dipping a small offcut of the scouring pad found on the back of the combination washing up scourer/sponge and dip this in a small amount of the required colour. Then blot off the excess on a piece of kitchen paper or toilet tissue. have a few practice runs first....but all you do then is dab it on the vehicle in areas that wear. This method creates great realistic marks with practice.
Finish up with some bigger marking and rust stains with a brush and a little drybrushing if required and once fully dry and cured blend in with filters, washes and pigments as you desire.
Hope this helps as an example on a started model.

End result on a beat up vehicle

John