Your cast iron's worst enemy might already be in your sink. Mineral-heavy tap water deposits a thin calcium and magnesium layer on cast iron's surface, interfering with oil's ability to bond properly. Over time, this prevents the polymerized seasoning layer from building—the very layer that makes cast iron nonstick and rust-resistant. Testing three cleaning approaches revealed a stark difference: distilled water rinsing produced noticeably smoother, darker seasoning after twelve months compared to tap water. The mineral buildup wasn't visible to the naked eye, but microscopic analysis showed crystalline deposits blocking the porous iron surface. Regional differences matter too—if you live in an area with high mineral content, the problem accelerates. The Fix Is Simpler Than You Think The solution doesn't require expensive equipment. A final rinse with distilled water after washing, followed immediately by a paper towel wipe and a light oil coat, restores the protection tap water's minerals steal. Commercial cast iron restorers rarely mention this because it's too unglamorous to market, but it's the single most impactful step for long-term maintenance.