In many of my maritime paintings, I like to paint the background in light values with soft edges. This, of course, adds perspective and gives the viewer a feeling for the atmosphere. As a watercolor artist, one of my most frustrating challenges has been painting a soft edge, with control and confidence, in one swipe of the brush. Many times I've had to work on a dried edge by scrubbing and lifting to produce the desired effect. But nothing is as clean and artful as getting it right on a single pass of the brush.
 
Here is a technique that works and takes little practice.
(1)   Saturate the paper in the area to be painted. Continue that saturation about ¼ inch beyond the edge boundary.
(2)   While the paper is still "shiny" wet, blot the water up to the boundary of the desired edge.
(3)   Then paint the shape. However, only apply the paint up to 1/8" from the blotted edge. The paint will wick into the remaining 1/8th inch. However, it gently stops at the blotted edge.
(4)   This makes the edge soft, controllable, and predictable.