Described herein is an aqueous basecoat composition including a silane-based additive as hardener/self-crosslinking material to improve adhesion of the basecoat to the under—and overlying coating layers, as well as the cohesion property of the basecoat itself. Also described herein is a mixer system for preparing an aqueous refinish basecoat composition including at least one base color component, at least one pigment-free component, at least one hardening component including the silane-based additive and optionally at least one rheology module as well as a method to prepare an aqueous basecoat composition from said mixer system. Further, described herein is a method of coating a substrate with the basecoat composition and a clearcoat composition and jointly curing the basecoat and clearcoat composition as well as to a coated substrate obtained from the method. Finally, described herein is a method of using the silane-based compound as hardening additive in aqueous basecoat compositions.