I have to do a cake that has wood grains all around and on top, client does not want any fondant the one I was looking at is using an airbrush I bought an airbrush although I havent used it, any suggestions?

I have never used an airbrush before, but I have seen on a show where they took bC colored brown, covered the cake after the crumb coat and then froze it (i think) for a short time. When they took it out, they rubbed the brown covered part and it looked like wood. They where making a treasure chest. It was a segment on Famous Cakes.
I might not be exact, but maybe you could look it up or experiment. It looked amazing!
Blessings~