High velocity oxy-fuel (HVOF) thermal spray processes are used in applications requiring the highest density and strength not achievable in most other thermal spray processes.  However, HVOF process is not able to apply fine powders less than 10μm via a conventional powder feeder like other thermal spray processes.  The advantages of using smaller and nano-sized particles in HVOF process include uniform coating, less defect microstructure, higher cohesion and adhesion, fully density, lower residual tensile stress and higher deposition efficiency.  A new process has been developed to realize HVOF spraying of finely structured layers by feeding liquid precursors of metallic fine particles. Process investigation has shown the benefits for making single and multiple layered coatings with full density and high bond strength attributing to the very high kinetic energy of particles striking the substrate surface and the improved melting of the small particles.  One of the targeted applications is fossil-fired boiler water walls that operate in a high temperature and hostile chemical environment.  The new coating system based on materials selection, structure design and process innovation, is promising to provide better protection of the boilers against various types of degradations like corrosion, oxidation, wear and interfacial failure.