2xxx series has a very different anodization curve than 6xxx series, the high Cu content of 2xxx doesn't help either. You should avoid anodizing different alloys together, this will complicate things. I'm not sure why this is, but LCD tolerates the traditionally "hard to anodize" 2xxx and 7xxx series better than conventional anodizing.
Try the experiment I suggest above, if we can isolate the problem we can fix it quicker and more effectively. It will be worth the effort.
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