The watercolor technique Flat wash is a process for painting large, solid-color backgrounds or underpainting for glazing and for smaller areas and objects in a composition using the wet-on-wet technique.
A sufficient amount of watercolor paint and water needs to be mixed ahead of time so that the color value is consistent throughout the flat wash process. If you have to stop to mix more watercolor with water, the paper will become dry, and your wash mixture will most likely not have the same color value as your initial wash.
For covering large areas, use a one-inch flat brush or an oval “wash brush” (also called a “mop brush”). To paint smaller areas with a flat wash, use a round brush size 10 or larger.
Controlling the brush and the painting speed is important. The wetness of the paper needs to be consistent until the desired area is covered with the flat wash. The tip of the brush should lightly touch the paper and move across the paper in a smooth, horizontal stroke.
When the flat wash is complete, leave the paper tilted at a slight angle on a board or easel until it has dried completely. Then proceed with painting the rest of your watercolor composition.
Recommended
Use 140 lb cold press archival 100% cotton rag watercolor paper. It is advisable to stretch the watercolor paper first to prevent it from buckling if you plan to cover the paper with a wet-on-wet flat wash. To avoid stretching the paper, I recommend using a watercolor paper block.