Painting and drawing with the Art Media tools : Using the Palette Knife tool
 
Using the Palette Knife tool
The Palette Knife tool interacts with the medium on the canvas. Generally, you use it with strokes placed by the Oil Brush tool as this is the only tool that applies large amounts of its medium. As the mass of the medium reduces in a spot, there is less to smear and using the Palette Knife tool has less effect. This means that the Crayon tool, which applies very little medium, is only slightly affected by the Palette Knife tool.
The method of loading or cleaning the Palette Knife tool is similar to the Oil Brush tool. You can clean it automatically after each stroke, retain the color picked up from the canvas, or allow manual cleaning. You can use the Palette Knife tool to apply pigment or smear it.
Paint strokes applied by the Palette Knife tool are wet in terms of how strokes interact with other strokes by smearing or mixing together. For more information, see To dry or wet an Art Media layer.
Palette Knife tool options
Edit workspace 
Palette Knife tool options include
Shape — defines the shape of the knife tip
Size — defines the size of the tool head in pixels
Thickness — defines the aspect ratio of the tool head
This setting is only active when the Fixed angle Head Tracking option is enabled.
Rotation — defines the angle of head rotation
Head Tracking — controls whether the tool head bends around the path of the user’s stroke, or remains at a fixed angle
Head Loading — defines the percentage of material on the brush at the start of the stroke
Auto Clean check box — cleans the tool and dips it into fresh paint at the start of a new stroke
Clean button — cleans the head and start the next stroke with fresh paint or pigment.
This button is only active when the Auto Clean check box is not marked.
Trace check box — allows the Palette Knife tool to select the pigment or paint color by sampling the data below the center of the tool regardless of the layer type.
Important! When you hold down the mouse button to begin the stroke, note that a single sample is performed and the resulting color is used for the duration of the stroke.