Also remember that in the desert, as the temperature plummets at night, condensation forms on AFV. This leads to rust streaks. Modellers often use oil paints to do these.
Try looking for AFV magazines - they usually do tutorials & as the most recent wars have been in the desert environment, you should be able to find one quite easily.
Just did a youtube search & found this
https://www.youtube.com/watch?v=ZVIMSXO6KJw