When you are not using your sail shade as a movie screen, I recommend taking it down and storing it in a safe dry place inside. This will help keep it’s white color longer, and it will make it far less likely to develop rips or tears.
While you can patch a sail shade that is being used for protection from the sun, when you are using it as a screen, you will want as pristine fabric as possible. Any dirt or smudges will appear on the movie. Sail shades can be cleaned with the right cleansers, but it’s easier just to keep it up while not in use.
If the shade gets a tear or rip, it’s just done, and you need to replace it. The ways of mending the shade will be exaggerated when they are hit with the light from the projector. Whether they create uneven parts of the movie, shadows or other blemishes, it will be annoying to anyone trying to watch the movie.