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.
We hypothesise that end-grain (transverse) wood surfaces will erode less during natural weathering than side-grain (radial and tangential longitudinal) surfaces. Douglas fir samples were exposed to 10 000 hours of weathering, and confocal profilometry was used to measure depth of erosion of the different planar surfaces. End-grain eroded less than side-grain, but the difference in erosion of end-grain and radial surfaces was only statistically significant if voids in end-grain created by checks were excluded from erosion measurements. Tangential surfaces were the most susceptible to erosion, due in part to the presence of lens-shaped voids caused by erosion of ray tissues. Erosion of earlywood was greater than that of latewood, particularly for end-grain. Erosion of earlywood is caused by thinning of cell walls in combination with transverse and longitudinal micro-checking, which creates loosely bonded cell wall fragments at weathered wood surfaces. We conclude that end-grain erodes less than side-grain surfaces when exposed to weathering, but differences in erosion of these planar surfaces is affected by checking, which complicates measurement of the erosion of wood using confocal profilometry. We discuss this issue, and describe the practical implications of our findings for the use of wooden posts as boundary markers for mining leases.
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