Synthetic skills are the prerequisite and foundation for the modern chemical and pharmaceutical industry. The same is true for nanotechnology, whose development has been hindered by the sluggish advance of its synthetic toolbox, i.e., the emerging field of nanosynthesis. Unlike organic chemistry, where the variety of functional groups provides numerous handles for designing chemical selectivity, colloidal particles have only facets and ligands. Such handles are similar in reactivity to each other, limited in type, symmetrically positioned, and difficult to control. In this work, we demonstrate the use of polymer shells as adjustable masks for nanosynthesis, where the different modes of shell transformation allow unconventional designs beyond facet control. In contrast to ligands, which bind dynamically and individually, the polymer masks are firmly attached as sizeable patches but at the same time are easy to manipulate, allowing versatile and multi-step functionalization of colloidal particles at selective locations.