Fly ash (FA), a common type of industrial waste, has garnered significant interest due to its high value-added potential. These micron-sized materials serve as inorganic fillers to enhance mechanical and shielding performance and have diverse applications in the coatings industry. However, research has shown that FA possesses high porosity and low interface-binding ability, which diminishes its barrier efficacy and can lead to the corrosion of the underlying metal. To mitigate this issue, a popular approach involves combining FA with nanomaterials or modifying the surface of the fillers. In this study, we utilized FA in conjunction with metal oxide nanoparticles (MNs) and performed surface modifications on the FA fillers with silica (sFA) and polyaniline (pFA), and on MNs with stearic acid (sMN). We then evaluated their impact on corrosion resistance, barrier, and mechanical performance. The findings revealed that coatings using unmodified FA and MNs showed minimal improvement in barrier performance during long-term exposure to a corrosive environment (3.5 wt% NaCl). In contrast, the surface modification of the fillers resulted in excellent anti-corrosion performance, demonstrating a five-order increase (10¹⁰ Ω.cm²) in coating resistance and ~69% less water uptake compared to the unmodified system. Accelerated corrosion tests indicated that coatings with surface-modified fillers could achieve up to 95% protection efficiency. The developed coatings exhibited robust properties, showing an increase in impact strength of about ~118% and an adhesion strength of about ~112% compared to blank coating, surpassing the performance reported in several other studies. The use of mixed fillers and their homogeneous dispersion in the coating matrix as a result of surface modification increased the diffusion path length for corrosive media by creating numerous tortuous paths, leading to enhanced corrosion resistance, barrier performance, and mechanical properties. This research underscores the utilization of high-value FA as a solution for solid waste management.