I’m not keen on your workflow for several reasons, primarily because it does not allow you to change any of the settings of your key to suit the background, which is often critical to creating a good comp.
If I were you, rather than exporting as a QT, I would either send to Motion from FCP, which would allow you continue to change any parameters of your key in FCP, or even better, forget the key in FCP and key in Motion.
If you’re dead set on exporting the QT, you have export as animation – millions of colors +, and you need to do that with the clip unrendered on the timeline to preserve the alpha channel.
David Roth Weiss
Director/Editor
David Weiss Productions, Inc.
Los Angeles
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