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Well, the robot may encounter a situation in which some expected piece in the metal is missing, and it is unable to spray the part properly because of this. To overcome this, the robot could use laser-light detection to first check if the part is in fact missing (or in the wrong position), and then either alert one of those icky flawed humans to manually take care of the outlier, or program it to spray around the problem. This would also help if there was some type of obstruction.
A second problem may be that the robot runs out of spray. As simple as it is, the robot can't very well make it's own spray, in which case the robot should be either hooked up to a larger supply of spray, or be programmed to sense the level of spray in supply, and to automatically stop the spraying process when it gets low enough, much like a printer would.