I recently painted (by brush) my corrugated iron roof with the colourbond colour - Woodland Grey.
The roof and mushroom head of the air-conditioner were originally Cottage Green. The air-conditioner installer (20 years ago) however never used colourbond sheeting for the air-conditioner stem. It was always raw galvanised sheet metal.
I primed this metal area with Dulux Metalshield High Build ZP primer in preparation to paint and then thought why not go the full hog and paint the entire air-conditioner with the primer and paint it all Woodland Grey?
Dulux couldn't advise me as no one had ever tried.
I painted small areas with just the primer, just the roof paint and then both the primer and the roof paint.
My conclusion is the primer sticks like glue to fibreglass and plastic. Just painting with the roof paint can be peeled off easily.
Primer and the roof paint together and after a few days it's almost impossibe to scratch off the roof paint.
I guess time will tell - but for the moment, I'm happy with it...