I have a slew of polished aluminum parts (covers) on many of my motorcycles and none of the is "coated". They all have retained the luster for years with the occasional buffing with polish.
Magnesium is a different story. It can be polished but needs to be coated, otherwise it darkens within days.
There are also quite a few anodized aluminum parts, e.g. wheels and engine covers and they do not show any reaction to conventional cleaners?
Well, the operative words are "occasional buffing and polish".
As for anodizing, almost all detergents and cleaners of most kinds are basic, or high pH. (yes, I know all about pH neutral cleaners, and Simple Green...) Some, such as wheel cleaners, are acidic and low pH. Alkalinity or high pH attacks fatty and oily soils breaking them into component parts that are easier to remove with surfactants. Acid or lo pH really attacks caked on and baked on soils, such as crusty brake dust on a wheel, and that "eyebrow" outside the wipe area on a windshield. The first I noticed the effect of alkaline on anodized aluminum when I apparently used an alkaline car wash product on my Pagoda, and the newly replaced clear-anodized aluminum trim
became a milky white. Metal polish cured that, and of course, between the initial car wash detergent and subsequent metal polish, the anodizing was gone. And yes, with "occasional buffing and polish" they remain clean, bright, and polished.
Back in October I restored my old Italian racing bike, and one of the needed tasks in restoration was removing the faded and worn black anodizing done to certain components back in the 1970s. The anodizing came off readily with a water based degreaser, ZEP purple. When I ran out of that, I used "Oil Eater". Same basic stuff without the color. You can see the before and after. All that black came off with a water based degreasing product.
