Painting a sky in acrylics follows the same basic principles of painting a sky with any other type of paint. With acrylics, you use water, rather than oil or turpentine, as a thinner for your paint. When painting a sky, remember to create a darker, more intense region of color at the top of the sky. The distant horizon should contain muted, pastel hues. Blending the darker and lighter regions with soft washes of water and paint creates a smooth, natural effect.
Paint pure blue across the top portion of your canvas. Use water to dilute the paint slightly in order to achieve a smooth appearance. Move your brush in horizontal motions.
Add a small amount of white to your blue paint. Apply this color in horizontal motions for a short distance down the length of the sky.
Make a mixture of one-half blue paint and one-half white paint. Apply this color to the sky in horizontal strokes until the painted region of the sky measures about two-thirds of the height of the region you intend to paint.
Add a small amount of blue to your white paint. Apply this color to the remaining portion of the sky.
Wash your brush and dip it into your water. Use the wet brush to blend the different tints of blue into each other to make a smooth transition between these regions.
Use your shoulder, not your wrist, to paint. Painting from the wrist creates a curve in your brushstrokes that will make sky appear to bulge in the center.