I get a lot of requests for information about how I make the clouds on my scenic backdrops. Here's the process:
The background starts as hardboard, Masonite or Gator board that's been painted blue. I found the blue color by holding a paint chip up to the sky on a bright sunny day and matching colors.
Next, I paint the background as shown below.
I have three layers - the blue color at the top, blue mixed with white halfway down and white mixed with a little blue at the bottom. There are no formulas. I just mix white with blue and paint it on knowing that if it's too light or dark I can fix it in the next step.
Here I painted in the distant mountains and then blended everything together using an airbrush with the high volume tip and flat white indoor wall paint.
Now for the clouds.
I use the Miles Hale method (published many years ago in RMC) and cut stencils from poster board. These are held about 2" in front of the backdrop and clouds are lightly misted-in using a spray can of flat white paint.
Here are stencil clouds sprayed on a module backdrop. You'll notice I've used several different stencils and overlapped them to get an interesting effect.
Here's the completed scene with the clouds in the background. Have fun!