Anisotropic building blocks are necessary for the self-assembly of complex structures. Methods are reported here for the preparation and characterization of patchy particles. Spherical polymer particles were temporarily bound to a substrate, so that part of their surface is occluded during subsequent surface modification by adsorption of polyelectrolyte. The resulting surface-charge pattern was detected, and its size measured, by means of selective nanoparticle adsorption to this surface. Feasible (roll-to-roll) production rates and process yields are also discussed. In the limit explored here of a single small patch, self-aggregation of patchy particles was observed only at high salt concentration, conditions that suppress anisotropic interactions. Complementary particles however exhibited site-specific binding, to form various anisotropic aggregates.