When painting any bare timber surface, the Australian Standard AS/NZS 2311:2017 Guide to the Painting of Buildings specifies 1 coat of a quality acrylic or oil-based primer be applied, followed by two coats of a quality acrylic or oil-based topcoat. Some topcoats for exterior timber surfaces are self-priming (e.g. Dulux Weathershield) and are suitable to use as a 3 coat system, but using a quality primer as the first coat is considered best practice for evening porosity, and blocking tannins and stains. Unpainted rough sawn timber boards will use 30-40% more paint than face dressed (smooth) board.
Pre-primed timber weatherboards are coated with a thin, holding primer that is only designed to provide temporary protection during the building process. This primer is generally not considered to be part of the paint system and should be sanded and coated in an appropriate prepcoat as shown here. For pre-primed timber weatherboard, please refer to the timber manufacturer’s specifications.