A composite structure includes a substrate with pores of a first mean pore size and a coating on at least one surface of that substrate. This coating has pores of a second mean pore size where the first mean pore size is equal to or greater than said second mean pore size. When the pore size of the coating is effective to capture particulate greater than 0.2 micron, the composite may be formed into a filter effective to remove microbes from a fluid medium. One method to form the porous coating on the substrate includes: (1) forming a suspension of sinterable particles in a carrier fluid and containing the suspension in a reservoir; (2) maintaining the suspension by agitation; (3) transferring the suspension to an ultrasonic spray nozzle; (4) applying a first coating of the suspension to the substrate; and (5) sintering the sinterable particles to the substrate.