As a pro painter can you afford anything other than bare metal unless you are very sure of the paint history?Steve..
I've no problems painting over factory paint, but do strip back when the paint is suspect or if funds do not allow, I would use a barrier coat to prevent reactions.
Not much I can do with peeling paint though other than strip it all off.
The paint stripper removed the most recent coat and another beneath and that revealed another shiny silver finish beneath that so it looks to have been painted a few times with no prep between.
You can tell 95% of the time if the existing paint is OK to go over but if in doubt, strip it off.
The paint on this roof felt and looked fine, it also sanded nicely. It wasn't until I blew the dust away that it began peeling. There was no sign of it prior to that.
Cheers
Ben