The primer will stick to bare metal very well, IF the metal is clean but we want epoxy first.
In production shops, they do use it for a ring of metal around bodyfiller work and have been for years but I explain the risks.
Personally, I don't even like the 4-5 inch areas but a lot of production shops just cannot take the time to spray the epoxy first and I do understand that.
Best way to put it, is like one of our dealerships put it to me a few weeks ago.
They do what we just talked about all day long with 3 painters and about once or twice a year they have one that comes back with a bubble, they always call when this happens and like the manager said to me, well one comeback out of 500 is still more cost efficient then waiting for epoxy to dry.