<p>This paper proposes a novel density-based topology optimization method for designing coated structures with variable coating thickness and smooth boundary representation. In addition to the standard density design variable, the filter radius of an improved PDE-based filter (which is integrated into the classical two-step filtering and projection procedure) to simplify the modeling, is selected as an additional design variable and optimized to achieve the non-uniform coating. After getting the pre-optimized result with non-uniform coating thickness using the structured mesh, a post-processing optimization stage is applied to refine the structural boundaries. Specifically, a level set surface is constructed based on the coating identification field, enabling body-fitted mesh generation via the Mmg platform, followed by further optimization of design variables. Considering the compliance minimization problem, sensitivities of the objective function and constraints with respect to design variables are derived, and several 2D and 3D numerical examples illustrate the benefits of considering variable coating thickness and the effectiveness of the proposed method for designing coated structures.</p>