As I stated in my previous comment WheresCarmen, the masking method works,
unless the objects you are masking contain text, then there are a few more
steps necessary to make it work correctly. The easiest method is to do
essentially the same thing, but make a large rectangle that covers everything
but the stage, and place it on the top layer. This is much less processor
intensive as well, especially if you have a lot of moving objects that need
masked.