I frequently create an alpha channel on a still graphic in Preview, using the magic wand. Sometimes when I do this and import the still into FCP X the alpha channel work but the still image is semi-transparent too. There's no indication of bad selections in Preview.
I've checked all the settings in the FCP X inspector and all seem normal.
If I duplicate the image on the timeline and stack it over the original it appears solid. So, any ideas as to what I must be doing wrong? Maybe in Preview?
That's a nice trick in Preview. I didn't know about that. So I can't help you there. But I've been playing with Pixelmator and I like it a lot as a Photoshop replacement. Only $30 or so in the app store. You'd have a bit more control.