|Paint Application Principles and Techniques
"Wet-on-wet" is a variety of direct painting in which brushstrokes are applied on top and worked into paint beneath them while both layers are fresh and wet. The wet-on-wet technique allows you to mix colors directly on the canvas. The technique also enhances fluid, spontaneous brushstrokes since a wet layer underneath allows a newly loaded brush to to flow easily across the surface. Use a soft hair brush and practice applying different amounts of pressure to the brush. More pressure will help blend colors; a light touch will deposit paint on top without lifting the underneath color up into the new stroke. Don't go over the brushstrokes again unless you want to blur them. Use a longhaired bristle brush to paint wet-on-wet with more pronounced brushstrokes and blending between colors.
An extreme of "painterly" application is impasto, which is a buildup of thick paint. Some painters practice a technique in which impasto is built up everywhere across the painting. Others focus the impasto on areas of greatest illumination and other focal points. Because an area of impasto is actually a relief with volume, the paint catches light along its high ridges and casts shadows into the valleys of paint.
To create impasto, you can mix your oil paint with heavy-bodied mediums such as Oleopasto or oil painting wax. Or you can apply oil paint straight from the tube, with no medium or thinner, in order to take advantage of the thick, buttery texture that is characteristic of oil paint in its normal tube consistency. Apply the paint with a stiff bristle brush or palette knife to add extra marks onto the thick paint. For a rich mixed color effect, try loading the brush with two colors at once. Thickly applied paint will hold its shape, so it is important to pay attention to the direction, width, and length of the marks you are making. If you don't succeed in creating the shape of brushstrokes that you want, use your knife to scrape the paint off and try again.
Sgraffito involves scratching through an outer layer of wet paint to reveal a different color (applied in an earlier, now dry, layer) of the primed support underneath. The scratching may be done with a wide assortment of instruments, such as the blade of a palette knife or the end of a paint brush. Don't use an instrument that is too sharp, or you may damage the support.
Drybrush is a method for applying paint that creates and effect that looks like uneven flakes of color with another color showing underneath. Using a stiff bristle brush, lightly apply a small amount of "dry" paint across the surface of the painting. (Use opaque paint, no medium and a brush that is beginning to "run out" of paint.) The drybrush technique works best across a textured surface. The dry paint attaches only to the upper ridges of paint, allowing the previously applied color to remain visible in the valleys. The result is a sparkling effect of diffuse, broken colors.
To obtain the necessary textured surface, use a canvas with a heavy weave (or add sand to the gesso); or wait until late in the painting process when underlying layers have dried and built up a thick surface.
Some painters prefer to eliminate evidence of brush marks. One strategy is to use a medium containing stand oil or to use a copal varnish as a medium--these tend to level out all brushstrokes, thus enhancing a smooth final surface. Another strategy is to stroke lightly with with a soft brush, such as a fan blender, back and forth across the borders of separate brushstrokes, thus creating a seamless surface without any impasto. This blending technique is effective when you want to model forms by creating a smooth transition between adjacent colors or tones. Beware of overblending, however, or the transitions may become too vaguely defined. You may want to blend only selected areas of your painting, seeking a more textured effect elsewhere.
[from Robertson and McDaniel, Painting as a Language, pp.34-35]