Artists use edges to establish focus and to take advantage of the beautiful painterly qualities of watercolor. Look at any scene, and consider what you are focusing on and what you are not focusing on. You see the sharp edges in your area of focus, while the edges in the area what you are not focusing on will be blurred.
Sharp edges draw attention to an area. Look at a painting filled with hard edges. There is an impression of looking at a series of cutouts. Each item may appear to be on the same level or plane. Each of the shapes fight for attention. You can limit your hard edges and thereby establish the focus of the painting.
If you are looking at a photograph or painting from life, squint at your reference. The distinction between similar values, even with different colors, will disappear. That can give you some direction on an area where the edges should be lost.
Also look at your photograph or other reference. You want more harsh edges where you want the view to focus. If you see a harsh edge, consider whether you want the viewer’s eye to go to that harsh edge in your painting. If not, than consider how you will lose that edge, and perhaps change the values.
Lost edges take advantage of the characteristics of watercolor. You are not coloring a coloring book, painting within the lines. You are creating a painting and want to use all the qualities of watercolor to their advantage. Let colors blend into each other. In so doing, you will be creating and mixing different colors on the paper, not on your palette, increasing the vibrancy and interest your painting.
In my painting, Leather Feather, if I had created a harsh edge for the hat, I would have created a secondary focus on the hat that would fight with the face and draw the viewer’s attention. Therefore, I lost the edge of the hat, creating a more effective painting.