## 3 Results and Discussion
### Light-Induced Colour Changes in the Coated Wood Surfaces
For the directly exposed areas (F2/V2/Z1) (Figure 4A), the evolution of the global colour changes, expressed by the \(\Delta\)E values, showed a quite similar pattern for map and ash, with changes occurring rapidly during the first 24 h of exposure (3.34 units for maple and about 2 units for ash), followed then by a slower increase up to the 5.23 units for maple and 3.49 units for ash. The colour changes for the walnut samples evolved somehow differently, almost linearly in time, reaching the highest value of 5.53 units after 72 h of light exposure. A trend of darkening expressed by negative \(\Delta\)L* values was registered for all wood species after the first 24 h of light exposure, but this trend was reversed by prolonged exposure. After 72 h light exposure, a slight increase in lightness was registered for walnut  
Figure 4: Comparative in-time evolution of the light-induced colour changes (\(\Delta\)E, \(\Delta\)L*, \(\Delta\)a*, \(\Delta\)b*) for maple, ash and walnut wood surfaces coated with waterborne acrylic lacquer F2 (samples type V2), depending on the exposure situation: (**A**)—direct exposure to UV light (F2/V2/Z1); (**B**)—exposure through a 1 mm clear glass lamella (F2/V2/Z3). (\(\Delta\)L* = +0.96 units), while the \(\Delta\)L* values for ash (\(-\)0.12 units) and maple (\(-\)0.48 units) remained slightly negative but very close to zero. From a chromatic point of view, the variation in redness Aa* and yellowness \(\Delta\)b* depended on the wood species. An increase in redness \(\Delta\)a* was determined only for walnut (2.16 units), the values for ash and maple being close to zero. Positive \(\Delta\)b* values indicated for all the coated wood species a continuous yellowing, following species dependant patterns, up to 3.45 units for ash, 4.96 units for walnut and 5.11 units for maple. To conclude, the colour stability of the wood samples coated with the acrylic water-based lacquer decreased in the following order: F2-ash > F2-maple \(\cong\) F2-walnut. This is different from that observed for the uncoated wood samples (walnut > ash > maple) and those coated with the acrylic-polyurethane finish F1 (F1-ash > F1-maple > F1-walnut), pointing out that both the wood substrate and the coating film influence the colour light stability of the coated wood. The light-induced colour changes for the same coated wood surfaces, covered with 1 mm thick transparent glass (F2/V2/Z3 areas), plotted in (Figure 4B), were again smaller than those determined for the directly exposed areas (Figure 4A), the influence of the covering glass being also dependant on the wood species. The highest decrease in the global colour changes (\(\Delta\)E after 72 h of exposure) was observed again for maple (about \(-\)3 units), while the lowest was observed for walnut (about \(-\)0.5 units) (Figure 4B). According to the graphs plotted in (Figure 4B-B4)), light exposure through the covering glass influenced mostly the evolution of the yellowness changes \(\Delta\)b*, which were reduced for all species, with a more pronounced influence (about \(-\)3 units) for the maple and ash samples. It is important to highlight that the order of colour stability among the F2 coated wood samples was slightly changed by the effect of the glass cover (S), so that a clear difference could be observed between walnut and maple: S-F2-ash > S-F2-maple > S-F2-walnut. Finally, the light-induced colour changes for wood surfaces coated with the polyurethane water-based lacquer (1 K), coded F3, are presented in Figure 5 comparatively for the directly exposed areas (F3/V2/Z1) (Figure 5A) and areas exposed under a glass covering slide (F3/V2/Z3) (Figure 5B). For the directly exposed areas (F3/V2/Z1) (Figure 5A), the evolution of the global colour changes, expressed by the \(\Delta\)E values, show more specific wood species patterns. For ash (coated F3), fast and important colour changes occurred during the first 24 h, a maximum \(\Delta\)E value of approximately 4 units being registered after this period.