Here’s some copy-paste from the thread in the B6/7 S4 section talking about this
Dparm - If the oxidation is heavy, WD40 won’t work.
B6Joe - Also, the OP said he cleaned his roof rails which are bare aluminum as far as i know. this wd-40 trick will NOT work on the window trim. this is because the window trim is clear-coated and the aluminum oxidizes under the clear coat. to remove the oxidation on the window trim, you must first strip/sand away that clear coat and then polish and re-clear it. or just leave it bare and polish as needed
SquiddyB6 - The aluminum trim on this car, and indeed many, many cars on the road, is not “clear coated”. A clear coat typically implies a paint of some sort. The aluminum trim is anodized with a clear layer. This is why it lasts so well, this is why it’s so hard to remove the finish and get to polishing, and this is why it looks like it does when it starts wearing out. The clear anodized layer is very hard, which makes it hard to remove. But the material underneath is thin, so going at it rough is necessary, but it takes a sharp eye to know when you’ve removed the anodized layer and have started removing aluminum. And the microcracks in the anodized layer are why it looks like it does when it starts looking bad. You may put some surface treatment on it, like an oil, to help fill the cracks, but the cracks are still going to be there. The only real, honest fixes are to replace the trim with new or used in good shape (very expensive), strip the anodized layer and reanodize (very expensive), strip the anodized layer, polish, and seal with something less permanent than anodizing (high-maintenance), or continue applying bandaids.
Now, maybe the band-aid method works for a long time, and that would be great, as I’m sure many of us have or will have this problem before long, but I’m just preparing you for the long term. Pretty much all shiny trim pieces these days are aluminum, not chrome, and are hard anodized clear, and not “clear coated” with some form of paint or sealant.
Dparm - Probably easier to just remove the clear anodizing and let it be bare metal. It would require some regular maintenance (polishing), however, or it will tarnish.