Abstract
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.
Highlights
Grain raising, the lifting of fibres when water is applied to sanded wood surfaces, increases the roughness of wood, and is mentioned as one of the reasons why companies in North America are reluctant to finish wood products using water-borne coatings [1]
In low-density wood species, such as balsa and western red cedar, “fibres” are thin walled and are perforated and shredded during sanding resulting in the formation of slivers of cell walls that are loosely bonded to the underlying wood surface
We have shown that there is an inverse relationship between the density of wood species and grain raising, and have suggested a mechanism to account for this relationship
Summary
The lifting of fibres when water is applied to sanded wood surfaces, increases the roughness of wood, and is mentioned as one of the reasons why companies in North America are reluctant to finish wood products using water-borne coatings [1]. Grain raising can be eliminated by lightly sanding wood to remove “fibres” projecting from surfaces [2,4], or its severity can be reduced by using modified finishes containing resins or hydrophobes [5,6,7,8]. The first paper on grain raising by Koehler in 1932 used reflectance microscopy to examine the surface of planed or sanded mahogany (Swietenia sp.) and oak (Quercus sp.) woods, before and after wetting [9]. Koehler’s images clearly revealed that increases in roughness of wood surfaces following
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