MIG Productions makes some great finely ground pigments that come with instructions on how to use them, but missing-lynx has some instructions too: http://www.missing-lynx.com/rareworld.htm
How to do rain marks, washing, filter (?), chipped paint and pigments. Downside (if you are dirt poor) the MIG bottles cost around 3-5 dollars a bottle.
They even make an Iraq sand pigment, perfect for your LAV-25
P.S. Welcome to our cozy nook we call Finescale forums! lol