I use brush chipping as my go to method, I've found that its less messy and that it works great. I've spelled it out in the Armor Forum, but here goes:
1-Paint a dark base paint or primer, let dry
2-Brush on AK Heavy Chipping with any type of brush to the area(s) you want chipping to occur, if you
don't want to chip that spot later, no harm. Let dry overnight
3-Spray on your top coat and let dry.
4-Take a round brush and trim it down to about an 1/8 of an inch
5-Use clean water and start using this brush to add water to the area and keep rubbing. Theres some
variables here but at some point the top coat of paint will start wearing away showing the base coat to the areas you want the effect.
And thats it
Heres one that I used it on.
Terry