In negative painting, the edges are mostly hard and defined. If some soft, blended edges are needed for variety, the already-painted edges may be feathered with a soft, slightly wet blending brush. By thinning gouache with water, a thin wash can be painted over any part of the underpainting. These edges may then be feathered with a blending brush. A 3/4-inch (19mm) red sable bright is ideal for softening edges.
–Jo Taylor, author of Watercolor Wisdom