## 2 Materials and Methods
### Characterization  
Initially, the morphology of the bio-based additive derived from rice bran wax was examined using a low vacuum scanning electron microscope (SEM JEOL IT 300, JEOL, Akishima, Tokyo, Japan). The analyses were conducted using an acceleration voltage of 15.0 kV and operating at a working distance (WD) of 10.0 mm. Furthermore, energy-dispersive X-ray spectroscopy (EDXS, Bruker, Billerica, MA, USA) was employed to analyze its chemical composition. Additionally, SEM was employed to examine cross-sections of the coatings, offering insights into how various additives and their concentrations impact the structure of the composite layers. To assess the visual characteristics of the coatings and illustrate the influence of additives on their overall appearance, colorimetric analyses were conducted using a Konica Minolta CM-2600d spectrophotometer (Konica Minolta, Tokyo, Japan) with a D65/10\({}^{\circ}\) illuminant/observer configuration in SCI mode. Gloss measurements following the ASTM D523/14 standard [57] were performed using an Erichsen 503 instrument from Erichsen Co.Fo.Me.Gra Instruments (Milan, Italy). A total of 9 assessments of color and gloss were performed on 3 samples per series, with 3 measurements per sample. Furthermore, the aesthetic influence of the additives was evaluated through surface texture analyses of the coatings using the MarSurf PS1 mobile surface roughness measurement device (Carl Mahr Holding, Gottingen, Germany). To assess the long-term aesthetic performance of the bio-based additive under harsh conditions and the protective efficacy of the UV absorber, samples underwent accelerated degradation tests in a UV chamber for 48 h. The UV chamber used was a UV173 Box Co.Fo.Me.Gra (Co.Fo.Me.Gra, Milan, Italy), following ASTM D4587-11 standard procedures [58]. This includes exposure to UV-B radiation (313 nm) at 60 \({}^{\circ}\)C. During the exposure period, colorimetric and gloss analyses were conducted to assess the coatings' visual uniformity and investigate the influence of various additives. Likewise, the heat resistance of the paint was tested by analyzing the color and gloss changes in the samples exposed to severe thermal variations simulated using the climatic chamber ACS DM340 (Angelantoni Test Technologies, Perugia, Italy). The exposure test followed the UNI 9429 standard [59] and involved 15 cycles, each consisting of:  
* 4 h at +50 \({}^{\circ}\)C and relative humidity below 30%;
* 4 h at \(-\)20 \({}^{\circ}\)C;
* 16 h at room temperature. To prevent moisture absorption by the poplar wood substrate, silicone was applied to seal the five untreated sides of the 40 \(\times\) 40 \(\times\) 2 mm\({}^{3}\) samples. This sealing procedure was conducted before the experiments to maintain the integrity of the wood substrate. The impact of additives on the barrier properties of the coatings was initially evaluated using a liquid water absorption test, conducted following the EN 927-5:2007 standard [60]. Five uncoated surfaces of 40 \(\times\) 40 \(\times\) 2 mm\({}^{3}\) poplar wood panels were effectively sealed with silicone to prevent water absorption into the wood substrate. Prior to testing, the samples were conditioned at 65% relative humidity and 20 \({}^{\circ}\)C. They were subsequently immersed in a container filled with water, and the amount of moisture absorbed, measured in grams per square meter (g/m\({}^{2}\)), was recorded at intervals of 6, 24, 48, 72, and 96 h. Three samples per series were included in the test. Additionally, the visual appearance of the samples was tracked during the test using colorimetric measurements and gloss analysis. In a similar manner, the test for resistance to cold liquids was performed to assess how well the coatings could endure exposure to different chemical substances, in accordance with the UNI EN 12,720 standard [61]. For this examination, filter paper (weighing 90 g/m\({}^{2}\)) was immersed in separate solutions containing 15% sodium chloride, pure acetone, olive oil, and coffee.