*2.1. Wood*

Top surfaces of the naturally dried and planed Norway spruce (*Picea abies* Karst L.) boards were gradually ground along the grain on a belt sander with 80 grit and 120 grit sandpapers, and then freed from wooden dust with compressed air. Following this, the samples: (a) with a dimension of 80 mm × 45 mm × 8 mm (axial × radial × tangential) for the artificial weathering test, and (b) with a dimension of 375 mm × 78 mm × 20 mm (axial × radial × tangential) for the outdoor weathering test were prepared from boards. Selected samples, i.e., without biological damages, juvenile wood, and growth inhomogeneities, were conditioned at a temperature of 20 ± 2 ◦C and a relative air humidity of 65 ± 5% achieving their equilibrium moisture content of 12 ± 2%. The density of the spruce samples used in the experiment ranged from 450 to 475 kg·m<sup>−</sup>3. The roughness parameters *Ra* (arithmetic mean deviation) and *Rz* (arithmetic mean of the heights and depressions of the profile at the basic length), determined for twenty selected samples perpendicular to the grain using the profilometer Surfcom 130A (Carl Zeiss, Jena, Germany) in accordance with the standard EN ISO 4287 [50], ranged in the intervals of 6.04–7.42 μm and 50.2–57.6 μm.

#### *2.2. Fungicidal Pre-Treatment of Wood*

For the fungicidal pre-treatment of spruce samples we used 2 wt.% water solutions of boric acid (H3BO3) (Funchem, Czech Republic) and benzalkonium chloride (BAC) belonging to QACs, synonym: alkylbenzyldimethylammonium chloride (Sigma Aldrich, Germany). Parameters of the soaking technology were as follows: a pressure of 100 kPa, a temperature of 30 ◦C, and an immersion time of 10 min. Spruce samples pre-treated with fungicides were then again conditioned achieving their equilibrium moisture content of 12 ± 2%.

#### *2.3. Plasma Modification of Wood Surfaces*

The top surfaces of spruce samples were modified with atmospheric low-temperature/cold plasma in laboratories of the Slovak Technical University in Bratislava, Slovakia, using the "Atmospheric Discharge with Runway Electrons" (ADRE) device (EST, Tomsk, Russia). The plasma modification was performed for one half of each group of spruce samples—the native, H3BO3 pre-treated, and BAC pre-treated. The following operating conditions were set in the ADRE device: a power of 1.4 kW, a frequency of 2000 Hz, and an exposure time of 60 s (Figure 1).

**Figure 1.** Plasma modification of the top surfaces of spruce samples in the "Atmospheric Discharge with Runway Electrons" (ADRE) device.

The initial contact angles γ0 of redistilled water with the top surfaces of the natural (fungicidally un-pre-treated) spruce samples were measured using the SEE System (Advex Instruments, Brno, Czech Republic) instrument, always for 10 replicates. The contact angle γ0 for the native unmodified spruce samples was 55◦ ± 2.3◦, and after the plasma modification it actually decreased to

13◦ ± 1.6◦ as a result of the polar functional groups created in wood surfaces; however, after 24 h, it partly increased to 20◦ ± 5.5◦ and after 72 h even to 35◦ ± 6.5◦—these tendencies are in accordance with [45].

#### *2.4. Coatings and UV-additives*

In experiment two types of transparent coatings synthesized by the company Chemolak Smolenice Ltd., Slovakia were used: (a) the acrylic water dispersion with a solids content of 28.6% (density 1.05 g/cm3), and (b) the alkyd in non-aromatic petrol with a solids content of 54.9% (density 0.91 g/cm3). Two UV-additive types were added into these coatings in the amounts of 0, 0.25, 0.5, and 1 wt.%: (a) the hindered amine light stabilizer (HALS) containing bis-(1,2,2,6,6-tetramethyl-4-piperidinyl)sebacate and methyl-(1,2,2,6,6,-tetramethyl-4-piperidinyl) sebacate: commercial product EVERSORB 93, (Everlight Chemical Industrial Corporation, Taipei, Taiwan), and (b) the UV absorber containing three types of hydroxyphenyl-benzotriazole (BTZ): commercial product EVERSORB 80 (Everlight Chemical Industrial Corporation, Taipei, Taiwan).

#### *2.5. Painting of Wood Surfaces with Coatings*

Before painting, the spruce samples were conditioned for 24 h at 20 ± 2 ◦C after plasma modification. This downtime was chosen exactly because with prolongation significant chemical changes in the plasma modified wood surfaces can occur, following by decreases in their hydrophilicity and wettability (see Section 2.3), [45,51], and also certainly in their adhesion with coatings. The pneumatic spraying technology was used for the coating's application in three layers—each layer in an amount of 150 ± 10 g per m<sup>2</sup> of the wood surface. The second layer and the third layer of coatings were implemented after a 24 h intermission needed for the drying and curing of the previous layer applied to the wood surface and for its subsequent sanding with 240 grit sandpaper.

#### *2.6. Weathering of Coated Wood*

The 1 week artificial weathering of coated spruce samples was performed in the Q-SUN Xe-1-S Xenotest (Q-Lab Corporation, Westlake, OH, USA). It took place in accordance with the modified version of standard EN 927-6 [52], i.e., 24 h conditioning of samples at 45 ◦C and then 48 subcycle steps—each lasting 3 h (2.5 h UV-radiation and then 0.5 h water spraying). These changes to [52] existed: Xenon lamps instead of fluorescent UV lamps; irradiance at 340 nm set to 0.55 W·m<sup>−</sup>2·nm<sup>−</sup><sup>1</sup> instead of 0.89 <sup>W</sup>·m<sup>−</sup>2·nm<sup>−</sup>1; the temperature on the black panel at 50 ◦C instead of 60 ◦C.

The outdoor weathering of coated samples took place from May 12 2018 for periods of 0, 14, 28, and 42 weeks. It was performed in accordance with the standard EN 927-3 [53], at a slope of 45◦ in metal stands oriented to the South, located in the industrial town of Zvolen, Slovakia, at 300 m above sea level. The testing area is characterized by many foggy days, smog, and high-temperature di fferences between summer (to 35 ◦C) and winter (to −25 ◦C). Mean climatic conditions in this area were as follows: a temperature of 9.4 ◦C; a relative air humidity of 83%; water precipitation of 700 mm/year; and sun irradiation of 1100 kWh/m2.

#### *2.7. Adhesion between Coatings and Wood*

The adhesion strength between the individual coating types and the individually pre-treated/modified spruce samples was determined with the pull-o ff test for adhesion in accordance with the standard EN ISO 4624 [54], using the PosiTest AT-M Adhesion Tester instrument (DeFelsko, Ogdensburg, NY, USA). A bond connection between the surface of the coated spruce sample and the steel-roller dolly with a diameter of 20 mm was made by two-component epoxy resin. The pull-o ff test tensile strength method measures the tensile force perpendicular to the phase interface "coating—wood" system, requiring the tearing o ff of the steel-roller from the coated spruce sample, at which the failures could occur either in the weakest interface "coating—wood" (usually typical in this work), or in the weakest component "coating" or "wood".
