I gave a friend an old steel frame, he primed it, and left it in primer. He built in into a nice bike. He never rode it in the rain, it was stored in the garage. The top tube is very rusted right through the primer.
It will rust if there is exposed steel under the primer. I have done a couple of frames myself, and coated the primer with finish coat. They're fine a decade later. They do get ridden in the rain, but always completely dried off before being put away. They look the same.