You can freehand the soft edges of the mottle, but you need to experiment on an old model to figure it out. Other systems for softening edges involve using masks with raised edges, using for instance Blue-tack or tape rolled into worm shapes and spraying over the edge to let the edge soften. I usually freehand, but with low pressure and a fine spray, doing the outside of the mottle first and then filling in the center. It's time consuming, but gives good results.