Region-aware coating-load selection for waterborne acrylic finishing of flattened bamboo panels based on adhesion performance and interfacial stress analysis

Conventional coating-load optimization for bamboo finishing is commonly guided by film build or whole-surface adhesion, which may obscure local weak regions in structurally heterogeneous flattened bamboo. To address this limitation, this study proposes a region-aware and weak-zone-oriented coating-load selection framework for flattened bamboo panels using a fixed one-primer–one-topcoat waterborne acrylic system. Four representative regions, namely outer-node, outer-internode, inner-node, and inner-internode regions, were coated at 60, 80, and 100 g·m⁻². Dry film thickness (DFT), cross-cut adhesion, pull-off strength, substrate flexural properties, SEM observations, and comparative finite-element analysis were integrated to clarify how coating load regulates film formation, adhesion response, and interfacial stress development across structural regions. The mean DFT increased from 50.61 μm at 60 g·m⁻² to 60.77 μm at 100 g·m⁻², but this increase was not proportional to the nominal coating-load increase and did not lead to a linear adhesion improvement. Pull-off strength was significantly affected by structural region and by the interaction between structural region and coating load. The inner-internode region showed a non-monotonic response, increasing from 0.11 ± 0.03 MPa at 60 g·m⁻² to 0.47 ± 0.07 MPa at 80 g·m⁻² and decreasing to 0.22 ± 0.05 MPa at 100 g·m⁻². Finite-element analysis further indicated that the experimentally weakest region did not correspond to the highest simulated interfacial shear stress; instead, adhesion sensitivity was governed by the combined effects of local stiffness, porous interfacial structure, coating build, and deformation compatibility. Within the investigated range, 80 g·m⁻² was identified as a weak-zone-oriented balanced load rather than a universal optimum, improving adhesion in the structurally sensitive inner-internode region while avoiding the greater thickness variability and stress-transfer demand associated with 100 g·m⁻². This work shifts coating-load optimization from average-performance maximization toward structurally informed weak-zone control, providing a practical basis for value-added finishing of flattened bamboo panels.