An apparatus and method of preparing and coating a large structure such as a ship's hull (21) while in a dry dock wherein a plurality of spray guns (22) disposed in an array are positioned by a robotic arm (27) in a spaced relationship along the surface to be treated so that their spray patterns overlap are disclosed. The array of spray guns is traversed downwardly a thus painting a trip whereupon the spray guns are secured, move horizontally and then activated to be moved upwardly until another strip adjacent to and overlapping the first strip is painted. Travel of the system along the work surface is accomplished by a reference track, which may be virtual or actual along which an unmanned platform (20) travels. An articulated computer controlled arm is carried by unmanned platform, which in turn carries the assemblage. Other tools may be selectively operatively connected to the arm for cleaning the hull before a coating is applied thereto.