The Scumbling technique, whether you are using oils or acrylics, opaque or transparent, is very much the same process. The idea is to build layers of broken color on top of each other to give a texture and luminosity to the painting. It is similar to the dry brush technique, where you dip your brush into paint and then remove a good amount of the paint with a rag. However, unlike dry brush where you create brush strokes, in Scumbling you jab the brush into a substrate (canvas, paper, board, etc.) causing the bristles to spray out leaving broken bits of color.
In this process, you will use a stiff bristle brush; I would recommend a Round to start. You should start with no more than three warm colors and three cool colors. Work with the first color, scumbling the paint onto the surface and allowing it to dry. Use the same process with a second color and let it dry. Try not to use too much paint on the brush, wiping off the excess paint if needed. Repeat the process again with another color, stabbing and twisting color over the dry colors. You should work slowly and not apply too much paint at one time. Remember, if your brush is too wet, wipe the paint off. It is also good to have a test surface to practice on until you see that the paint is being applied correctly, then paint on the original work.
When this process is done correctly, it can produce beautiful texture with optical mixing of paint fragments. Scumbling can be done with opaque or translucent paint. The multiple layers and colors showing through each other can create a sense of depth and luminosity to the paint. This lesson will require practice and you will have to experiment with painting in this technique.
Below is an example of a scumbled surface.
Examples: Master painters using a scumbling technique.
Claude Monet, Impression, Sunrise, 1872, oil on canvas, 19"x 25".