## 3 Results and Discussion
### Light-Induced Colour Changes in the Coated Wood Surfaces
polyurethane lacquer F3. values of the uncoated and coated specimens was tested by one-way Anova and Tukey's HSD as post hoc test, and this information is included as superscript codes. Based on the comparative \(\Delta\)E values, it can be observed that all the three coating materials F1, F2, F3 protected to some extent maple and ash wood substrates against the light-induced colour changes. This protective effect was higher for ash than maple. In the case of walnut, some protective effect was registered only for the coating F3. No effect was registered for the coating F2, while the samples finished with the coating F1 suffered colour changes slightly more intense (statistically significant) than uncoated walnut wood. Differences in the protective effects of various waterborne clear coatings have been reported before. For instance, an increased protective effect of a 2 k polyurethane topcoat (based on hydroxyl-bearing polyacrylate and aliphatic poly-isocyanate) compared to an acrylic topcoat was reported for Radiata pine boards primed with a moisture curing iso-cyanate [58]. Colour change values \(\Delta\)E varied in the range of approximately 2-10 units after 700 h light exposure (accelerated aging test simulating natural light passing through window glass) for spruce samples coated with four different three-layer coating systems (primer, middle layer, top layer) with waterborne coatings based on acryl/alkyl or polyurethane resins and their mixture [59]. The coated maple and ash samples exposed to light darkened less compared to the uncoated ones, as reflected by the comparative negative \(\Delta\)L* values for all the three finishing materials. In the case of maple, this effect appeared to be highest for the F2 coating material, while in the case of ash, all the three coatings seemed to have a similar effect. Contrarily, the coated walnut samples became lighter following 72 h light exposure, this effect being  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Wood Species**} & \multicolumn{4}{c}{**Colour Changes after 72 h Light/UV Exposure**} \\ \cline{2-6}  & & **Uncoated** & **F1** & **F2** & **F3** \\  & & **V1/Z3** & **V2/Z1** & **V2/Z1** & **V2/Z1** \\ \hline \multirow{6}{*}{Maple} & \(\Delta\)E & 9.36 \({}^{\text{A}}\) & 5.71 \({}^{\text{B}}\) & 5.35 \({}^{\text{B}}\) & 4.92 \({}^{\text{B}}\) \\  & & (1.71) & (1.20) & (2.11) & (1.64) \\  & \(\Delta\)L* & \(-\)5.07 \({}^{\text{C}}\) & \(-\)3.83 \({}^{\text{B}}\) & \(-\)0.11 \({}^{\text{A}}\) & \(-\)3.56 \({}^{\text{B}}\) \\  & & (1.61) & (0.84) & (1.18) & (1.76) \\  & & 1.48 \({}^{\text{B}}\) & 1.09 \({}^{\text{B}}\) & \(-\)0.14 \({}^{\text{C}}\) & 2.60 \({}^{\text{A}}\) \\  & & (0.79) & (0.58) & (0.66) & (1.45) \\  & & 7.58 \({}^{\text{A}}\) & 4.03 \({}^{\text{B}}\) & 5.11 \({}^{\text{B}}\) & \(-\)0.52 \({}^{\text{C}}\) \\  & & (1.38) & (1.49) & (2.30) & (1.48) \\ \hline \multirow{6}{*}{Ash} & \(\Delta\)E & 8.39 \({}^{\text{A}}\) & 3.37 \({}^{\text{B}}\) & 3.94 \({}^{\text{B}}\) & 2.25 \({}^{\text{C}}\) \\  & & (1.28) & (1.37) & (1.24) & (1.02) \\  & & \(-\)4.13 \({}^{\text{B}}\) & \(-\)0.52 \({}^{\text{A}}\) & \(-\)0.48 \({}^{\text{A}}\) & \(-\)0.51 \({}^{\text{A}}\) \\  & & (1.13) & (1.74) & (1 more pronounced for the coatings F1 and F3 and minimal for F2. This particular behaviour of walnut is due to its darker colour compared to the other two species, respectively, to the role of extractives and visible light in the occurring colour changes, as reported in the literature [18; 20; 21]. The statistical significance of wood species (1) and type of coating (2) as influencing factors on the light-induced colour change values \(\Delta\)E was verified by an Anova two-factor with replication test, at a significance level \(\alpha\) = 0.05. According to the summary results presented in Table 6, respectively, the \(p\) values lower than 0.05, it can be stated that both the wood species (factor 1) and the type of coating (factor 2) are statistically significant influencing factors.