When softwoods are dried and subsequently impregnated with waterborne preservatives two problems frequently occur, preservative screening of multi-salt preservatives and difficult re-drying. To study the causes, the permeability of rimu and radiata pine wood has been measured along the three principal directions. The radial and tangential permeabilities of the green sapwood of the two species were similar and of the order of 10-16 m². The transverse permeability of the green rimu intermediate wood was lower and of the order of 10-17 m². After drying and resaturation the radial and tangential permeability of the rimu intermediate wood were practically unchanged, while the transverse permeability of the rimu sapwood dropped to that of the intermediate wood. For radiata pine sapwood the tangential permeability decreased to 10-18 m². In contrast, after drying and resaturation the radial permeability of radiata pine sapwood was of the order of 10-14 m², two orders of magnitude greater than for the green wood. This increase in radial permeability was caused by an interplay of flow along the radial and axial resin canals. It is believed that the absence of preservative screening in radiata pine sapwood during impregnation is due to rapid dispersal of preservative solution along the resin canals, followed by movement into the tracheids where the preservative fixes to the cell walls. This mechanism cannot operate in rimu wood, as it does not have resin canals, and preservative screening occurs.

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