Photoshop :: Background Removal

I have a photo open on CS4, that I want to remove the sky and replace it with another. Since the photo has lots of "gridwork" in it (it's the Eiffel Tower in Vegas) I used Mask pro to remove the blue sky. When I save it back to CS4, is there a way to easily select the blank background so that I can insert the new sky? I can make a Quick Selection of the majority of the sky but again, it's the sky, or in this case now, the blank background peeking through all the grid work that is the problem getting selected.