**4. Conclusions**

Transparent coatings on wood outdoors do not have su fficient service life and colourfastness due to more significant penetration of sunlight into the coating film and the base wood. Another significant factor causing degradation of coatings in humid environments are moulds. The possibility of increasing the durability and resistance to mould attacks in acrylic and oil coating on beech and spruce wood was investigated. Initial treatments of wood by dipping in a 2% solution of ca ffeine and various concentrations of commercial FN-NANO ® containing TiO2 nanoparticles and their mixtures were used. A significant positive e ffect of ca ffeine treatment on the service life of the acrylate coating system during accelerated artificial weathering was observed in both types of tested woods. The 15% FN-NANO ® dispersions treatment also showed good e fficiency. The combined e ffect of a mixture of ca ffeine and FN-NANO ® had no significant positive e ffect. No improved quality of the oil coating was observed using the initial wood treatments. Degradation by moulds was significantly reduced when the wood was initially treated with a 2% ca ffeine solution. Higher concentrations of FN-NANO ® dispersion had a positive impact—in particular, on samples degraded for 6 weeks in a UV chamber. Based on the results, it is possible to recommend the initial treatment of spruce and beech wood with a 2% ca ffeine solution or 15% solution of FN-NANO ® dispersion containing TiO2 nanoparticles in order to increase the overall service life of transparent acrylic coating in exterior applications.

**Author Contributions:** The work presented in this paper is a collaborative development by all of the authors. Conceptualisation: M.P. and K.K.; methodology: M.P.; investigation: K.Š., O.D., P.Š., and D.N.; resources: M.P. and K.K.; data curation: O.S., D.N., and K.Š.; writing—original draft preparation: M.P.; writing—review and editing: K.K.; visualisation, K.Š. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the Czech Science Foundation, Project No. 19-02067S "The effects of methylxanthine-based biocides on the properties of constructional timber" and by Grant "EVA 4.0", No. CZ.02.1.01/0.0/0.0/16\_019/0000803 financed by OP RDE.

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