What you describe is very typical of a cowl. Unless there are holes due to rust around the lower vents that require repairs, it is probably best to leave it be. There are about a 100 spot welds holding the upper and lower cowl sections together, and this is a major PITA to remove and weld back together correctly, one, if not the most difficult, jobs on a Mustang. Some have cut sections on the side of the cowls to access the interior, using a painting wand for rust conversion and paint, or to fix minor rust holes around the vents.
These cowls were never painted inside and what you are seeing is overspray from various painting events.