What is it?

This defects looks like bare spots, that have been imprinted into the zinc coating of the steel. It is also known as dross or bare spots. The defect is caused by non-metallic particles that fall onto a partly-solidified zinc surface. If the particles are loose, they can be blown off again, which results in bare spots.

Oxide Flakes (on coated products)

Oxide Flakes (on coated products)