Using a painting knife to create textures in watercolor requires practice because timing is critical. But with a little experimentation, you can learn to use a painting knife in one of two ways: additive or subtractive.
One additive method involves pulling the thin edge of the knife through damp paint so that water and pigment fill this groove. Another technique is to dip the edge of the blade into paint and drag the knife across a dry surface. Both methods will create sharp, delicate linear textures. To use a subtractive technique, pull the palette knife toward you like a squeegee to squeeze the wet pigment from the paper, lifting off excess paint with a paper towel or leaving the paint to form shadows. Holding the knife at an angle suggests wood or rock textures. Use the flat tip of the knife to create soft lines.
You’ll have to experiment to learn when the paint is just dry enough to work with and how much pressure to apply on the knife. And keep in mind that these techniques can damage the paper, so be gentle. Once you’ve mastered these styles, experiment with the knife to find your own favorite angles.