Accurate coating thickness measurement is vital for maintaining product quality, ensuring corrosion protection, and meeting strict industry standards. This comprehensive guide explores the physical principles behind non-destructive testing (NDT)—including magnetic induction and eddy current methods—while breaking down common measurement errors caused by substrate geometry, surface roughness, and environmental factors. Readers will gain actionable troubleshooting strategies, best practices for gauge calibration, and insights into eliminating operator error to ensure reliable dry film thickness (DFT) readings.