After doing a lot of research, and talking to my local hardware store owner (close friend), this is the primer I have decided to use on the galvanized wall tin:
https://www.sherwin-williams.com/homeowners/products/extreme-bond-primer
I bought the last two cans the local store had in stock....and they were running a 30% off sale month of April! Sweet!
It is a Urethane Modified Acrylic bonding primer. Will even stick to glass, so after cleaning with dilute ammonia, this stuff ought to stick like white on rice! Going with a very light tan top coat....