Exterior wood coatings. Part-1: Performance of semitransparent stains on preservative-treated wood

Pressure-treated lumber available for residential applications had been changed from chromated copper arsenate (CCA) treatment to Cu–amine formulations (since January 2004). This change created a need to investigate the performance of coatings on these new Cu-based preservatives as compared with CCA. This study evaluated performance of a number of semitransparent deck stains on CCA, alkaline copper quat (ACQ), copper azole (CA), and untreated wood samples during 3 years of natural weathering. Coatings were evaluated for their ability to reduce water uptake, color change, checking of wood, and mildew growth and to improve general appearance of preservative-treated wood samples. Overall, Cu–amine-treated wood had on average 50% higher water uptake than CCA-treated wood samples. Although solvent-based coatings had better water repellency performance on CCA-treated and untreated wood, there was no significant difference in water-repellent effectiveness (WRE) between water-based and solvent-based coatings on ACQ- or CA-treated wood. All preservative-treated woods had less color change and better visual ratings than those of untreated wood samples. Based on image analysis which was used to determine the total area of surface checking on samples after 3 years, preservative treatments had no significant effect on reducing checking of wood while coatings reduced surface checking by 30–40%.