**5. Conclusions**

Wood represents a very complex and versatile natural material that can be modified in many ways to prepare it for direct use or even to activate its surface for further treatments. Modern treatments are physical (mainly plasma and densification procedures), chemical, and enzyme assisted or enzymatic grafting of di fferent functional molecules to the wood surface, and application of thin films (coatings) and deposition of nano-sized particles by sol-gel techniques. Some of the surface modification methods have been introduced a long time ago and are still in use, other strategies of modification have been developed in the recent years.

All these pathways have the main goal to improve or even enlarge the range of properties of wood, such as good adhesion of glues and surface finishes, hydrophobic behavior and/or better hygroscopicity toward water and waterborne formulations. An improved resistance of wood surface against weathering processes when exposed to outdoor conditions is attained by grafting functional molecules onto cell walls and treatment with nanoparticles. Environmental benefits conferred by many wood surface modification technologies recently developed include reduced toxicity of the process, increased wood products life, and reduced maintenance cycles, lower negative impact on the environment and carbon footprint of the products. The resistance against wood decay microorganisms remains a significant topic for such investigations.

Nowadays, an interdisciplinary approach to increase the life of modified wood products used in outdoor and indoor applications (claddings or structural applications) is known as the "four Ds" rules of design and these are: deflection (shedding of water as defense way), drainage (fast removal of water as essential hazard), drying (restoring the optimal conditions as a prerequisite reaction) and durability (improved properties, mainly decay resistance, fire resistance).

As related to the wood surface modification strategies using esterification reactions, the use of natural compounds and their derivatives may represent a feasible trend that can be successfully implemented without major alterations of present technologies. Polysaccharides (such as starch, cellulose, hemicelluloses and lignin-carbohydrate complexes, maltodextrin), ca ffeine, propolis and other bee products derivatives, etc. are of high interest. The wise choice of treatments will remain a key step as more methods under development are progressing from laboratory scale to industrial scale production. Even combinations of di fferent suitable methods (for example, applying a physical treatment prior to the esterification that would subsequently occur under mild conditions) are under research as they have the potential to maximize the overall e ffects.

**Funding:** This research received no external funding.

**Acknowledgments:** We dedicate this work to the memory of our colleague and friend, Ruxanda Bodîrlău (6 April 1953–26 June 2016), and thank her for her discretion, dedication and enthusiasm for as long as we have been working together.

**Conflicts of Interest:** The authors declare no conflict of interest.
