I'm done erasing and here's the final product. No noise in the sky and detail back in the rest of the image.



Another way to selectively apply a noise reduction filter is to use a selection mask. I will press the letter "Q" to enter "Quick Mask Mode" and in this mode I can paint a selection mask. Everything that is "not" painted red (I actually paint with black to get red mask, if I make a mistake I can paint with white to remove the red mask) will be selected. I will fill the sky with red and then invert the selection so that only the sky is selected.

I'm done painting my mask, so I'll hit "Q" to exit "Quick Mask Mode".



Everything, but the sky is now selected, but I want only the sky selected, so I will "Invert" the selection to select the sky. I can do this by going to the "Select" menu and and sliding down to "Inverse".