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