Utilizing lightweight Al alloys in various industrial applications requires achieving precise pressure tightness and leak requirements. Vacuum pressure impregnation (VPI) with thermosetting polymers is commonly used to address leakage defects in die-cast Al alloys. In this study, the efficacy of the VPI technique in sealing alloy parts was investigated using a combination of nondestructive micro X-ray computed tomography (micro XCT) and a standard leak test. The results demonstrate that the commonly used water leak test is insufficient for determining the sealing performance. Instead, micro XCT shows distinct advantages by enabling more comprehensive analysis. It reveals the presence of a low atomic number impregnates sealant within casting defects, which has low grey contrast and allows for visualizing primary leakage paths in 3D. The effective atomic number of impregnated resin is 6.75 and that of Al alloy is 13.69 by dual-energy X-ray CT. This research findings will contribute to enhancing the standard VPI process parameters and the properties of impregnating sealants to improve quality assurance for impregnation in industrial metals.