The silver-looking Ductwork above your air handler’ that goes through the ceiling and into the attic is called a supply plenum. Your supply plenum is made out of 100% Fiberglass. See circled pic.
If you live in a single family home than you most likely have distribution boxes in your attic that are 100% Fiberglass. See pic.
The can which is the piece the register or grill screws into is lined with fiberglass around every vent where the cold air comes out is Fiberglass (see pic).
The coating absorbs into and bonds with the fiberglass, when dry there will be a hard white shell that the air will travel through versus the previous fiberglass. See the before pic.
See after pic: