In a coating, line a first substrate such as a vehicle body, made of a steel plate, is coated separately from a second substrate made of a different material including a plastic, such as vehicle parts, i.e., bumpers and so on, and the parts are assembled with the vehicle body. The vehicle body and parts are coated separately to form each a highly reflective surface coating. The coating is implemented by a spraying step for spraying the paint on them to form a coat; and a drying step for drying the coat. The paint is sprayed in the spraying step in a film thickness which is thicker than a thickness at which the paint sags on a surface extending at least upwardly and downwardly. And the coat on each of the substrates is dried in the drying step while being rotated about its horizontal axis for a period of time ranging at least from before the paint starts sagging to until the paint achieves a substantially sagless state, rotation of the substrate being carried out at a speed which is high enough to rotate the substrate from a vertical position to a horizontal position before the paint coated thereon substantially sags due to gravity yet which is low enough so as to cause no sagging as a result of centrifugal force.