Iam here to keep the weather out. i am clear unless washed. i am here so you can see sky. i am smooth unless there's a crack in me.
riddle with 6 letters

The answer is window. 

A window will be clear, except when it is washed, because the window cleaner temporarily smears the window until it is wiped away. When you look out the window to the outside, you will see the sky. A window will be smooth unless it has a crack in it.