The present invention relates to a process for applying optical brightening agent (OBA) to a sheet of paper or paperboard substrate. The process comprises the step of spraying an aqueous composition comprising an optical brightening agent onto at least one surface of a sized paperboard substrate to form a treated paperboard substrate. The treated paperboard substrate includes a layer of OBA on at least one surface of the paper or paperboard substrate in which the brightness of the treated paper or paperboard substrate is greater than the sized paper or paperboard substrate before treating thereof.