Cellulose-based materials such as paper, cloth, and wood are inherently the most vulnerable to moisture uptake, UV-induced degradation, and loss of mechanical strength, which restricts their use in outdoor and other harsh environments. To overcome these limitations, four benzoxazine monomers were synthesized from allyl phenol (AP) and eugenol (E) using two cycloaliphatic amines (cyclohexyl diamine and methylene bis(cyclohexylamine)) via a simple and cost-effective Mannich condensation route. The corresponding polybenzoxazine (PBZ) coatings were applied to cellulose substrates and subjected to continuous UV irradiation at 365 nm for 7 days to evaluate their durability. Comprehensive characterization including FTIR, thermal analysis, tensile testing, adhesion strength, surface wettability, and optical measurements was conducted before and after UV exposure. The coatings exhibited excellent structural retention and showed nearly unchanged thermal stability following irradiation. Notably, cycloaliphatic amine–based PBZs improved the tensile strength of cellulose substrates by up to four-fold and significantly enhanced hydrophobic behavior, yielding a water contact angle of approximately 137°. The coated substrates also maintained high optical transparency (> 95%) with negligible changes in absorbance after UV aging. In addition, the PBZ-coated cellulose demonstrated efficient oil–water separation, achieving > 98% separation efficiency and a flux of 9090 L m−2 h−1. These results underscore the strong potential of cycloaliphatic amine–derived PBZs as robust UV-resistant, hydrophobic, and high-performance coatings for cellulose and other related substrates used in weather-resistant applications.