Dry brushing is a nice way to put on some detail onto a prop. They can give an allusion of lines or shadows. The key to dry brushing is that you take as much paint off the brush as possible. Brush specific areas and then go into deeper areas to pound the pigment in. You should stay with blacks or dark browns when dry brushing to get the best effect. Also, you should also make sure to do a wash prior to dry brushing.