Once the base coat dries, apply the blue sky paint with a brush or roller. Let it dry.
Then working at a 45-degree angle, slap white paint over the surface using short, random strokes from a 4-inch-wide decorator brush. Pull the paint out until it’s thin in some areas, and overlay it in others so that you create an uneven color overall. If you wish, imitate the color gradation found in the sky by leaving the tops of walls and the centers of ceilings more blue.
Work the paint so that it has a mottled effect. This mottling begins to suggest where you should place the clouds.
Use a dry 2-inch-wide decorator brush tipped with the white paint to establish cloud shapes in the light sections. Use random, flowing strokes to smooth and blend the two colors. Be loose, and don’t think of cloud shapes as such; your goal is to create billowy shapes with a mottled effect. Follow up by adding more blue with a brush wherever you feel it’s needed to create the right balance. Let the surface dry.
The final finish. To complete the finish, mix 8 parts water with 1 part white paint to make a thin wash. Brush this wash over the entire surface, and let it dry. It has a softening effect that breaks up the blue and pulls everything together.