Methods are presented for applying a coating material to the particle collecting regions and surfaces of the stages of a cascade impactor after the impactor is assembled for use. The methods comprise generation of a multimodal droplet aerosol of liquid coating material and delivering it into the impactor. The modes of coating droplet aerosol are selected to produce droplets that span the range of cutoff diameters for the stages of the cascade impactor. The coating substance improves the trapping of particles on the stages. The methods limit the total amount of coating material applied and confine it to the regions of particle impact opposite the stage orifices, thereby reducing the possibility of chemical interference when analyzing a test aerosol.