## 1 Introduction  
The purpose of this research, conducted within the framework of the DorART Project at the Universitat Politecnica de Valencia (Spain), was to study, through experimental testing, the mechanical properties, efficiency, and stability of the most commonly used gap fillers in the restoration of wooden artworks, with particular attention paid to their compatibility with traditional water gilding techniques. Given the structural fragility and artistic complexity of gilded wooden heritage objects--such as polychrome sculptures and altarpieces--this study aims to identify filler formulations that not only perform effectively under mechanical and environmental stress but also interact appropriately with water-based gilding layers. The significance of this work lies in its contribution to the development of restoration materials that ensure both structural reinforcement and esthetic coherence while respecting the physical and chemical behavior of the original materials. The deterioration of wooden works of art is a common phenomenon, as wood is a hygroscopic organic material that is susceptible to thermo-hygrometric fluctuations, anthropogenic factors, fire, and biodeterioration, including wood-decaying fungi, all of which contribute significantly to its degradation [1]. Consequently, the conservation of such objects depends on multiple factors, especially the specific properties of the wood [2], a thorough understanding of which is essential for selecting the most appropriate intervention methods to ensure its long-term preservation [3]. The current state of research on the behavior of filler materials used in the conservation and restoration of cultural heritage has given rise to several experimental studies aimed at analyzing the fillers' behavior under artificial aging conditions in different contexts, although these studies have not specifically focused on wood. In the field of heritage restoration, experimental examples can be found for the treatment of works of art on canvas [4], bone [5], and metal [6], as well as wax sculptures [7]. These cases reflect the diversity of materials addressed in conservation practices and the ongoing search for appropriate restoration solutions. A paradigmatic and pioneering example of material testing on artworks with wooden supports is provided in the study of [PERSON] and [PERSON] (1988), who evaluated 33 different filling materials under conditions of high relative humidity [8]. Building on this seminal work, [PERSON] and [PERSON] (1998) analyzed commercial fillers based on acrylic and vinyl resins, which contributed to the evaluation of more accessible formulations for use in conservation [9]. Other advances include the study by [PERSON] et al. (2004), who evaluated combinations of epoxy adhesives for application on wood, highlighting the role of intervening layers in improving reversibility [10]. [PERSON], in his research on flexible adhesives for gap filling, emphasizes the need for filler materials to replicate the mechanical behavior of the substrate and remain stable over time. Based on his practical experience, he proposes a hierarchy of materials, with ethylene vinyl acetate (EVA) being the most suitable choice [11]. [PERSON] (2017), focusing on structural fillers for wood artwork, confirmed the hygroscopic compatibility and dimensional stability of cellulose-based materials [12]. More recently, [PERSON] et al. (2020) examined exterior adhesives combined with inert fillers, such as wood dust and glass microspheres, and showed that low moisture absorption and adequate drying are key to preventing shrinkage on wood substrates [13]. Complementarily, [PERSON] et al. found that expanded vermiculite-based composites offered excellent mechanical and hygroscopic stability for indoor heritage works, as well as chemical compatibility with wood [14]. Finally, in 2023, [PERSON] et al. confirmed that hydrophobic wood fiber/epoxy composites exhibit structural and hygroscopic compatibility with wood, reinforcing their potential as durable restoration materials [15].