Room-temperature vulcanizing (RTV) silicone rubber is used as a high-voltage insulator coating to enhance the pollution performance of conventional insulators, and the electrical performance of RTV coating depends on the several parameters. The present study shows the effects of the thickness on the leakage current, erosion, and hydrophobic performance of high-voltage insulator coating. Several different thicknesses of RTV silicone rubber were taken into consideration on porcelain substrates. Erosion and leakage current performances of the samples were evaluated by using inclined-plane tests (IPTs) based on the IEC 60587 standard. Eroded masses and fundamental and harmonic components of the leakage currents were investigated with respect to coating thicknesses. On the other hand, hydrophobic features of the samples were evaluated by using dynamic drop tests (DDTs) with regard to CIGRE TB442 documentation. Time to loss of hydrophobicity and the level of the leakage currents were investigated in DDTs considering the thicknesses.