## 1 Introduction  
The surface of each material or final product consists of a miniature of peaks and valleys, the size and distribution of which determine the surface properties of the material, such as roughness, texture, etc. The surface roughness is estimated in order to predict the surface behavior of the material during its application in various uses. As regards wood, roughness greatly affects its aesthetics and the structures in which it participates, and should be in line with the criteria and requirements of consumers in terms of quality. As regards wood veneer sheets, rough surfaces of veneers not only negatively affect the appearance of the finished products, but also affect manufacturing processes such as coatings and adhesion appliance, and adhesion strength, since they reduce the contact between them, resulting, according to the literature, in weak interactions between glue and wood and, therefore, low-strength properties of laminated veneer lumber, plywood and several other wood-based composites [1; 2; 3]. Due to its structure and anatomical features, the wood surface is a multidimensional and complex substrate, and its roughness is influenced by various factors such as the wood species (hardwood versus softwood), wood density and porosity (denser wood corresponds to lower porosity and smoother surfaces), annual rings' width, ratio of early wood to late wood, the log temperature during slicing/peeling and wood storage conditions (temperature, relative humidity), the moisture content, wood anisotropy, structure and types of cells and the kinetics of liquids-gases into its mass, as well as several mechanical and machine processing operations (saving, sanding, planning, etc.) parameters, such as the cutting means type, knife angle and marks per centimeter, cutterhead speed, tool wear, cutting direction (longitudinal, radial and tangential), etc. [1, 3, 4, 5, 6, 7]. [PERSON] et al. [9] revealed that steaming for 20 h is an ideal pre-treatment of veneers to provide smooth surfaces of both heartwood and sapwood. In general, the use of coarse-grained veneers can reduce the bonding quality by 1/3, compared to smooth surface veneers [10, 11, 12]. Less rough wood surfaces exhibit better performance in the application of finishing agents, more uniform distribution of adhesive, require much lower amounts of paint/dye to cover the whole surface, while the phenomenon of resin bleeding through the face veneer is avoided [11, 12]. Furthermore, according to the literature, the surface roughness of wood material decreases as the grit number of sandpaper increases from 60 to 240 [13]. Usually, the veneer production industries apply a sanding of 80-100 grit number, to keep the production cost at low levels, while the woodworkers and manufacturers further apply additional sanding processes to the veneer-based panels, once or twice, using 180 or 220 sandpapers. The defects generally affect the appearance of the veneer sheets, making them usually less preferable for face-side application in furniture and structures [3]. A high number of defects makes the veneer be categorized as low-value and it is usually applied in back-side applications. However, the wood defects correspond to the natural appearance of wood and, especially in recent years, there has been a phenomenon of asking for artificially aged or intensely rough furniture, precisely because they refer to and remind the customer of something special and unique. Furthermore, in the recent years in which the wood of high quality has been in short supply, the rational utilization of woody biomass, even the low-value wood species bearing a high number of defects, as well as the high-quality raw-lumber saving strategy, seems to be of crucial importance. Currently, there is no comprehensive information available concerning the way that several different wood defects affect the smoothness and surface quality of sliced veneer sheets manufactured from different hardwood species.