Thank you all for your help. Luma key and fade with a slug on top works for me.
I agree with Petter, FCP X must have white levels beyond 255. Is there any point in having that? I know that you can go beyond 255 if you work in HDR 32 bits. Can FCP X do that? I know that in AE you can do some fun stuff by animating HDR material… Anyway, that’s off thread. Thanks again. David