Microstructure and Mechanism of Grain Raising in Wood

Grain raising, the lifting of fibres when water is applied to wood surfaces, is a reason why some companies are reluctant to finish wood products with water-borne coatings. However, the elements that lift-up and cause grain raising have not been identified, and the relationship between wood density and grain raising has not been clarified. Our work sought answers to both questions. We planed or sanded different woods using aluminum oxide abrasive paper, and characterized surfaces using profilometry and SEM. Surfaces were re-characterized after wetting and drying. Grain raising is inversely related to wood density. In particular, very low-density woods are highly susceptible to grain raising, whereas grain raising does not occur in high-density woods or planed woods. In low-density woods, sanding tears cell walls creating loosely-bonded slivers of wood that project from surfaces, particularly after wetting and drying. This mechanism for grain raising was confirmed by modelling the action of abrasives on wood cell walls using an array of hollow tubes and a serrated tool. Less commonly, fibres and fibre-bundles project from surfaces. We observed that grain raising was correlated with the coarseness of the abrasive and conclude that it can be reduced in severity by tailoring sanding to account for the density and surface microstructure of wood.