Ed:
I will try the sony mask. Thank you.
However, I have tried applying the multiply mask to the smoke effect and putting the base clip as the child. When I do this the base clip (underlying clip shows black until it gets to the point underneath the smoke clip and then both clips show for the duration of the mask only. In other words when I apply the mask and click on the base clip to play in the timeline, it is all black except during the mask portion. Am I doing something wrong? Will it render correctly? I would expect the base clip to play all the way through and the mask to blend in for its duration, not black out the entire base clip.??