Three-dimensional printer fabrication is improved receiving a digital model of an object, the digital model specifying two or more different build materials; prioritizing the two or more different build materials with a ranking; identifying exclusive locations in the plurality of layers where a surface layer of the object has one of the two or more different build materials; generating an external structure for the object according to one or more design rules; matching a first build material of the external structure to a second build material of the object at each of the exclusive locations; and generating tool instructions for fabricating the object and the external structure using at least the first build material and the second build material.