Paper materials are composed of networks of fibres, and unlike in fibre-polymer composites, the mechanical loads are transferred through the network via fibre-to-fibre joints (or just fibre joints) rather than by a surrounding matrix phase. The mechanical properties of the materials are governed mainly by the network structure, the properties of the fibres and the properties of the fibre joints. Surface modifications using the Layer-by-Layer treatment improve the stiffness, strength and strainability of the materials. Here, the effect of such modifications is studied using a micro-mechanical model of a thin paper sheet. The investigation shows that the observed increase in stiffness and strength cannot be fully explained by changes to the joint properties or changes to the number of joints. To match the experimentally observed improvements, the fibre modulus must also be affected by the surface treatment. Hygroexpansion measurements confirm that the modified networks contain larger dried-in strains, indicating stronger drying restraints when the network first forms. This drying restraint on the microscopic length scale is known to cause an increase in the fibre modulus. The findings show that material improvements due to surface modifications are due to a combination of improved joint properties and fibre stiffening caused by drying under increased restraint. The surface treatment improves stress transfer between fibres through the increased joint area and the number of active joints, leading to a more effective distribution of drying-induced tensions throughout the network. However, the results indicate that fibre stiffening is the dominant mechanism behind the improved mechanical properties