For large areas I make a brush in photoshop that looks like a scatter of small dots, and change the spacing in the brush options so each dot makes a continuous line rather than a line of dots. Put a taper on the brush if you need to on Shape Dynamics/Size Jitter> Pen Pressure (you need a drawing tablet like a Wacom to use this.) You can use this brush with the brush, mixer brush, dodge, burn and smudge tools to make the large areas of fur. For details I just use a harder edge round brush at a small diameter.