Influence of Primer Layer Configuration and Substrate Heterogeneity on the Overall Interfacial Performance of Waterborne Acrylic Coatings on Flattened Bamboo

Flattened bamboo (FB) exhibits pronounced structural and chemical heterogeneity between outer and inner layers and between nodes and internodes. These variations critically influence its interfacial performance with waterborne acrylic coatings. This study aimed to clarify how primer layer configuration and substrate heterogeneity jointly affect the coating adhesion, hardness, and abrasion resistance of FB. Three coating schemes—one primer and one topcoat (1P1T), two primers and one topcoat (2P1T), and three primers and one topcoat (3P1T)—were applied to four types of FB substrates defined by layer and structural position. Adhesion, pencil hardness, and abrasion resistance were measured according to GB/T standards, complemented by surface roughness, contact angle, XPS, and SEM analyses. Results showed that substrate heterogeneity dominated coating behavior. The parenchyma-rich inner-layer internodes, characterized by higher polarity (O/C = 0.296) and rougher texture, exhibited stronger adhesion and superior abrasion stability, whereas the fiber-dense outer layer nodes, with lower polarity (O/C = 0.262), showed weaker bonding. Increasing the number of primer layers improved film continuity only when the substrate microstructure allowed sufficient primer penetration. The combined findings indicate that coating adhesion and wear stability are primarily governed by substrate composition and surface polarity rather than by coating thickness. These results provide scientific and practical guidance for optimizing primer application and surface preparation in the industrial finishing of bamboo-based decorative panels, while also highlighting the environmental and economic advantages of waterborne coating optimization for sustainable bamboo manufacturing.