Strange banding during export or render only. help!

I have no idea what's going on. I've tried everything, but I just can't seem to fix it.
So I have a light leak that I'm overlaying , and when I set the opacity to anything below 100%, this strange thing happens where I get crazy banding on the export or the render. The weird thing is, if i pause it in FCPX it's not visible at all, it looks flawless, and even when playing back in fcpx it's not too bad, but if i render it then watch it, or just export it, it's extremely bad. I've tried many different export settings, they don't change anything. I cannot for the life of me figure it out. Somebody help!

Here's what it looks like comparatively: imgur.com/a/TKqZE

Unfortunately, banding like this is cause by limitations like 8 bit color, and 4:2:0 chroma subsampling, so I don't think there's that much you can do, rendering and exporting in a higher grade of ProRes might get rid of the effect, but that will extremely limit your playback options.

I'd try experimenting with different blend modes, and/or adding some grain to the light leak clip before applying it, and/or adding some grain afterwards.

Hope this helps,
-Mark

Thanks so much. I'm afraid you're totally right. I'll try these things. This website is an awesome resource!

You could try adding a small amount of ‘noise’ to the scene in Effects > Stylize > Add Noise.

Set ‘Noise Type’ to ‘Blue Noise’ (that's important) and start with a very low setting, maybe .03 or .06 then try the opacity slider to vary the effect.

It’s a balance between reducing the banding and the noise becoming too obvious. Test the effect with the clip running rather than parked on a still frame.

John