Use a paint gauge before, during and after the sanding process in order to accurately measure how much clearcoat has been removed. Paint gauges measure the total thickness of the finish and when used, determine how much clearcoat has been removed during the repair process. Paint gauges range from magnetic pull type to sophisticated electronic types and are available from a variety of sources. The older magnetic type gauges, at best have a 5 percent accuracy range and are not sensitive enough to detect removal of 0.020 mm 0.5 mil; (0.0005 in) clearcoat. The newer type magnetic gauges have improved accuracy ranges. Most gauges are confined to checking either ferrous metal (steel) or non ferrous metal (aluminum) panel. At this time, there are no viable gauges for reading film thickness on composite (SCM Doors, RIM Fenders) panels. The more sophisticated (ETG) electronic paint thickness gauges digital type gauges are able to read film thickness on both ferrous and non-ferrous metal panel. Digital (ETG) gauges may have an accuracy range of 1 percent and include thickness standards for recalibration. The following paint thickness gauges are available. Call 1-800-GM-TOOLS for information: