Despite being able to cover a bodyshell in one heavy coat, probably without runs or sags the best and most reliable way, in the sense of the paint staying on the body is to spray a series of light, thin coats allowing each one to dry thoroughly before applying the next.
Bodyshells flex a great deal, especially in crashes and although the paint does have a strong bond to the surface if one thick coat is applied it will be more rigid than the body. Should the body have to flex excessively, say in a crash, the paint layer not being as flexible as the body will inevitably crack and the bond being broken, come off. Applying the paint in a series of thin layers nets a much more flexible covering which will be less likely to come off.