You can paint under the hood, engine compartment, inside trunk, and even the door jambs ahead of time. Almost everybody does. But for outside body panels that are going to be viewed from the same angle (i.e. top of hood & top of fender, or side of door and side of fender...) you will be much better off spraying them all at once. Unless you can exactly match the 100 or so variables that effect how the paint goes on, and how the metallic/pearl particles "lay", spray them all at once. If the paint were a solid color with no metallic, usually you can do whatever you want... Usually.