What works best is to dab arcrylic paint on pretty heavy with a brush, and then take a damp or dry paper towel or rag and wipe the paint, removing most of it. If you have something that water would be running down, wipe top to bottom. If you want the windswept look like your biker scout, wipe front to back. You end up removing most of the paint you initially dab on, but as you build up the layers of doing this process of dabbing and wiping off a few times, the results are quite realistic looking.
This is how I weathered my custom mando, and my pack and helmet turned out awesome.
