The ability to achieve multi-layer synthesis on the surface of microbeads is critical for diverse chemical and biological assays. Although microfluidic techniques for layer-by-layer (LbL) synthesis have been demonstrated for droplets, accomplishing continuous flow multi-layer synthesis for microbeads has remained a significant challenge. Here we present a micropost array railing (μPAR) system to achieve continuous flow LbL functionalization on microbead substrates in a microfluidic device. Microposts arrayed at a 1° angle served as a railing system for directing microbeads into adjacent flow streams of bio-molecules and washes. An extended biological linker (including four biotin-avidin pairs) was synthesized onto streptavidin-coated 15-μm diameter polystyrene microbeads via an eight-stage LbL process (with additional wash steps between each reaction). After completion of the biological linker, a fluorescently labeled analyte was bound to the linker and microbead fluorescence was observed.