A head unit for a surface-treatment apparatus is provided and includes a housing that includes a fluid inlet, a dispensation unit, and at least one baffle. The fluid inlet receives fluid from a fluid reservoir. The dispensation unit defines a dispensation chamber and an input port in fluid communication with each of the dispensation chamber and the fluid inlet. The at least one baffle is configured to direct the flow of fluid through the dispensation unit and out of the elongated dispensation outlet.