Polymers, such as polydimethylsiloxane, can be utilised as functional coatings with water repellent properties in granular materials. These new surface-modified granular materials have potential applications in barriers to control wetting. However, limited knowledge exists on the mechanical behaviour of particles with synthetic coatings. Here, a fundamental insight is provided by investigating the micromechanical behaviour of a coated sand with extreme water repellency by means of a custom-built inter-particle loading apparatus. To synthesize the water repellent coatings, an industrial sand was subjected to 3% mass ratio of dimethyldichlorosilane. In a comparative investigation between coated and uncoated (natural) sand particles, the results revealed (1) a decrease of the normal stiffness and (2) a dependency of the inter-particle friction coefficient on the normal force for the coated sand particles. In particular, the inter-particle friction coefficient significantly increased at low normal forces (lower than 2 N) with this effect decreasing as the coatings were damaged under a high normal force (over 5 N), and with the micromechanical behaviour of the coated sand converging towards that of the uncoated (natural) sand. The results are compared to theoretical models and mechanisms are proposed to explain the findings. By changing the nature of the coatings, this paper provides a basis for future research on granular materials with functional coatings for other applications. (C) 2019 Elsevier B.V. All rights reserved.