The black primer on the sheetmetal I got was very nice - uniform and bound tightly to the metal. The satin finish just needed a good scuffing.

The gray primer was junk, and I guess it was there just to keep the metal from rusting on the shelf. With minimal effort it would flake off. It should all come off before priming and painting.

Take an awl or similar and scratch an inconspicuous spot. If it flakes, strip it all off.