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I recently flew on an Airbus A321 and noticed these gray spots on the wing, circled in red. What are those?

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They are rivet ends. The paint seems to have a hard time sticking to those, not an uncommon sight on older planes.

Thanks to Darrel Hoffman's comment, I did some digging:

The phenomenon is known as rivet rash.

“Rivet rash” refers to selective loss of paint from aluminum rivet heads on in-service aircraft, as shown in Figures 1 and 2 and 3. Airlines are dissatisfied with this condition since it greatly detracts from the airplane’s decorative paint appearance in areas most visible to the passengers, namely the 41-section (nose) and entry doors. In addition, rivets can “rash” within six months of delivery giving the airplane a prematurely older appearance.

Quoted from Boeing Environmental Tech Notes: Rivet Rash (pdf)

A simplified explanation is that the rivets usually have a coating protecting them during storage and installation, or the properties of the rivet alloy itself may be different enough from the sheet material of the wings, that the adhesion of paint is compromized.

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    $\begingroup$ The real question is... what's the red circle for? Is it also common on older planes? $\endgroup$
    – Opifex
    Commented Jun 8, 2021 at 15:03
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    $\begingroup$ My guess is that the rivets cool at a different rate from the panels surrounding them, leading to uneven expansion and contraction of the paint. You see this on ground vehicles and structures as well if they're subject to extreme cold. $\endgroup$
    – Darrel Hoffman
    Commented Jun 8, 2021 at 15:39
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    $\begingroup$ @Opifex a survey of pictures of older planes in StackExchange question suggests they do indeed commonly have red circles on them. $\endgroup$
    – leftaroundabout
    Commented Jun 8, 2021 at 16:45
  • $\begingroup$ @DarrelHoffman extremely plausible. Once a small crack in the paint forms on the perimeter of the rivet head, it's bye bye paint very quickly, considering the hostile environment they have to endure. $\endgroup$
    – Jpe61
    Commented Jun 8, 2021 at 16:50
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    $\begingroup$ So it has a name, huh? I wonder if it's called something different outside of the aeronautics industry, because I've seen it on everything from buildings to boats to park benches, etc., but the term "rivet rash" appears to be exclusive to planes. Granted in some cases it might be nails or screws rather than rivets, but the effect is pretty much the same. $\endgroup$
    – Darrel Hoffman
    Commented Jun 8, 2021 at 18:42

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