Surface preparation has a profound effect on the rate of flow of fluid through wood, particularly in the longitudinal direction of flow. For best results, the surface must be devoid of any debris and/or obstruction. The use of a sharp, thin knife appears to be an effective way of preparing samples for natural permeability measurements.
Choong, E.T.; McMillin, C.W.; Tesoro, F.O. 1975. Effect of surface preparation on gas permeability of wood. Wood Science 7(4):319-322