Blubby, you should have spoken up when you first noticed the bare metal. Still, the installer should unbolt the unit and apply touch up paint. Since it's on the bottom where it can't be seen, Rustoleum should be fine. If it's on the bottom edge, the correct color should be used. I can't imagaine that the manufacturer doesn't supply touch up paint.
Dougfamous, you should change your registration to "unprofessional member". To quote my daddy, machinist and high-school graduate, "I wouldn't do business with you for a five cent bag of peanuts." Obviously this was long ago when American workers had pride.