We start with a pre-defined mask of the sky (see Masking video).
Invert the mask (Ctrl + Shift + I), to select everything but the sky.
We will use this new mask to apply a creative effect to the image.
Click the Image Sprayer tool, and then drag or click inside the mask to spray with the default image list.
Holding down Shift, drag in the image window to decrease or increase the nib size, and then continue to spray.
Press Ctrl+R to remove the mask.