I put a couple of drops of Tamiya acrylic flat black in a plastic paint palette and add enough water until it is the correct consistency. I brush it onto the part and use a paper towel moistened with water to wipe off the excess.
Cool truck! Very impressive scratch building especially on the camper shell! I am planning on building a scale replica of a base 1970 Chevy C10 that my Dad drove when I was kid.