Polyurethane coatings modified with silanized graphene and/or hexagonal boron nitride as nanoadditives were synthesized, characterized, and tested for ultraviolet degradation and hydrophobicity. These coatings containing various weight percentages (wt%) of nanoadditives were prepared and investigated. The coating composition was characterized using Fourier transform infrared spectroscopy. The silanization of nanoadditives was characterized using scanning electron microscopy. The glass transition temperatures (Tg) of the nanocomposite coatings were determined using differential scanning calorimeter; it was observed that the presence of nanoadditives in the coatings impacted the Tg indicating their interference with the polyurethane chains and structures. The change in the coating thickness and water contact angle after ultraviolet light exposure was also studied. Exposure of the coatings to ultraviolet light led to a decrease in the coating thickness and hydrophobicity. With the increasing content of nanoadditives, the decrease in the coating thickness was lower and the rate of decrease of water contact angle was slow. Polyurethane coatings with 0.8 wt% silanized hexagonal boron nitride nanoparticles exhibited minimum reduction in coating thickness and the slowest rate of decrease in the water contact angle.