Splaying the bristles.- This will give you interesting brush strokes. The way to do this is by pressing down on the brush, while turning it clockwise and counter clockwise until the bristles separate. Eventually if this is done many times, it will damage your brush. It is a good idea to keep one separately for this purpose. This comes in handy to depict clumps of grass, pine needles, many small twigs grouped together such as the edge on winter trees or small leafless bushes, to indicate the grain on barn wood, rusty tin roofs, etc.

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 Example of rust on a tin roof