What is a flat wash?
A flat wash is a basic watercolor technique that you will use nearly every time you start a new painting. You usually use it when you need to cover a large area of your paper with paint.
With the flat wash watercolor technique you end up with a flat or even area of color across the paper.
How do you create a flat wash?
Select a dark hue for your flat wash and mix enough paint on your palette to cover all of your paper. The more water you add, the lighter the color will be. To darken it, add some more color.
Fill your brush with paint, and starting in the upper left corner draw your brush across the paper to the upper right corner.
Fill up your brush again and paint across the next row, overlapping the first row of paint. Make sure to go over the damp edge of the previous flat wash, so that the wash is even.
Continue until you have covered all the paper. You can use his watercolor technique to create a basic flat wash as a background for your painting, or to create the sky or ground.