## 1 Introduction
Different principles for stabilising the colour of clear-coated wood have been in research focus: (i) pre-treatments of the wood substrate for an improved UV resistance, (ii) modification of the coating materials with additives for improved UV resistance and (iii) combined methods. Nanoparticles of TiO\({}_{2}\)[5], ZnO [37], CeO\({}_{2}\)[3], CH\({}_{2}\)O [38] or \(\alpha/\gamma\) Fe\({}_{2}\)O\({}_{3}\)[6], UV absorbers (benzotriazole, [39], benzophenone [40]), HALS (hindered amine light stabiliser), (Tinuvin 292, Tinuvin 293 [41], Tinuvin 5151 [39]), or chemical modification treatments with succinin anhydride [42], maleic anhydride [43] or soybean oil modified epoxy resins [40] were employed as pre-treatments. Organic UV absorbers and HALS stabilisers [34; 36] and inorganic UV absorbers and nanoparticles [6; 44] were employed as additives for clear coatings. Approaches combining the two principles were also tested [4; 13; 33; 39; 41]. Most of the studied references reporting light-induced colour changes in the coated wood surfaces refer strictly to the changes observed and measured on the directly exposed coated wood surface [1; 5; 17; 22; 45], without any information on the possible photo-degradation of the wood substrate under the coating film. However, colour changes in clear-coated wood may be caused by colour changes in both the clear-coating film and underlying wood, or either of them [35], and their contribution should be acknowledged and evaluated to deeper understand the occurring phenomena. References addressing this important issue are rather rare [35; 36; 46; 47]. Uncoated wood specimens and free coating films were employed to simulate clear-coated wood. A light reflection model was used to elucidate separately the colour changes caused by the clear-coating film and the underlying wood, showing a major contribution of the underlying wood compared to the coating film for two types of polyurethanes (approx. 60%/40%; 95%/5%) [35]. Different research concluded that in the case of spruce wood clear coated with a water-borne acrylic, the colour changes after 200 h UV irradiation were due almost entirely to the wood substrate, as colour changes in the coating film were negligible [47]. More studies have focused on the exposure of wood in outdoor conditions where UV radiation acts in conjunction with rainwater [2; 32; 45], while the topic of light-induced colour changes in indoor conditions by the natural light passing through the window glass [1; 22; 23; 36] or light from artificial sources [26; 27] has been less addressed, especially for clear-coated wood with natural aspect [46; 48]. All these point to the idea that stabilising the natural colour of wood species exposed to light in indoors conditions remains a challenge that could be better addressed based on a deeper understanding of the occurring phenomena and influencing factors. In this context, the objectives of the present study were: to evaluate comparatively the light-induced colour changes for uncoated and clear-coated wood surfaces with natural aspect in indoor conditions by accelerated testing; to determine the influence of the species and type of coating on the colour changes; and to highlight the specific contributions of the wooden support and of the coating films to the total colour changes in the finished surfaces. A dedicated laboratory method was employed for this purpose [46].