Just because it looks bare doesn't mean it's unfinished.

If I specified sheet aluminum for Type II Class 3 chemical conversion per MIL-DTL-5541, any materials QC professional would be hard pressed to visually determine that anything was actually done to the material.

To be sure, no, MIL-DTL-5541 isn't an uncommon spec constrained to military applications.

In addition to that, behind every beautiful, polished aluminum aircraft is a crew that is washing, cleaning, and polishing it regularly. Polished surfaces are higher maintenance than painted ones, generally speaking, and by quite a bit.

Polished aluminum is not low maintenance by any stretch.

I know I've noticed on airplanes before that while some of the wing is aluminum colored, some areas in particular are painted a vaguely aluminum-colored shade of grey.

And I think the... it seems they're called Krueger flaps? Those are often painted aren't they? And those I'd think you'd definitely want to de-ice.

On an aircraft that has a mixture of polished and painted surfaces, the painted surfaces often aren't bare aluminum. Often that's the wing root fairings, winglets, nacelles, radar cones, vertical stabilizer, and so on.

I don’t think I was meant to get a giggle out of this. Can you explain using other words? Of course a painted surface isn’t bare. It’s got paint on it.

Did you mean it’s not aluminum? Or a different grade?

Thanks, I laughed at it myself, in hindsight. I should have stipulated "underneath" and likely appended an "if you follow...".

Yes, well, they may either be materials that have other surface textures/coatings/treatments/finishes that aren't conducive to polishing, or they may be other metals, or non-metallic composites (can't put aluminum over a radar, for example), and so on.

TL;DR: The painted surfaces are generally instances in which polished aluminum won't really work, for a variety of reasons.

any idea what they use on aircraft skins?

That's not an application you can generalize. Literally everything is dependent on requirements.

> corrosion isn’t that much of a worry with aluminum airframes

The internal aluminum components are all finished with BMS 10-11 Type 2 primer (the ubiquitous green stuff). (Amazing what trivia I remember from 40 years ago!)

If you don't paint it, you have to polish it periodically, which is more expensive. A pre-pitted surface probably more so.

Planes that are freshly painted are 'slick' -- they have noticeably less drag! A good paint coating also makes it easier to wash dirt and other contaminants, which cause drag and corrosion.

Corrosion is indeed a problem, but only severely for salty places.

Paints also have UV screen for plastic composite components.

To the person who downvoted the above without leaving a comment:

Is this not the case? I know for a fact that planes whose home airport is near the ocean have more issues with corrosion.

If I'm wrong, I'd be genuinely delighted to learn how.

So everyone knows that aluminum develops a chemically stable oxidation layer that prevents ionization, or something like that.

In reality though, that layer shows zero mechanical strength. You don’t see that with aluminum products but only because they are always anodized. Without anodization or just left in the wild, aluminum develops white salt like corrosion on its skin.

I have anecdotal experience of this with a PowerMac G5. It was a school computer, I got to see inside, and years later I found fingerprints of my younger self permanently engraved on it.

That layer also doesn't shine, something people don't take into account. A local FBO with a fleet of gorgeous airframes polishes them at a minimum of monthly, and they don't fly more than about once a month.

Polished aluminum is labor-intensive to keep looking good. But I'm glad so many still take the time to do so.

didnt downvote you but your link is broken (lost 'g' at the end)

Whoops! Fixed, thanks.