Abstract: A coater head for applying coating to a size press roll of a size press coater, the coater head has a flow passage with a metering device at its outlet end and a metering chamber immediately adjacent to the metering end. A shear developing passage delivers coating to the metering chamber and a mixing passage leading into the metering chamber mixes fresh coating with return coating on the surface of the roll to carry the fresh coating to the outlet end of the metering chamber. The metering chamber is dimensioned to form a stable vortex within the chamber thereby to improve the uniformity of the coating wet film.