you can do the feathered thing with spraycans....
Cut stripes or shapes out of card and using blue tak, attach them to the area the mottle is to be applied and use a spraycan. A quick pass or two with the spraycan held vertically over the area to recieve the paint will give a good result. Light mist coats are best till the pattern builds up nicely.
By raising the mask slightly, you will get a nice feathered edge.
You can cut various patterns and apply the same technique as you go.
Hope that helps
Mike