Use a paint gage before, during and after the sanding process in order to accurately measure how much clearcoat has been removed. Paint gages measure the total thickness of the finish and when used, determine how much clearcoat has been removed during the repair process. Paint gages range from magnetic pull type to sophisticated electronic types and are available from a variety of sources. The older magnetic type gages, 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 gages have improved accuracy ranges. Most gages are confined to checking either ferrous metal (steel) or non ferrous metal, aluminum panel. At this time, there are no viable gages for reading film thickness on composite (SCM Doors, RIM Fenders) panels. The more sophisticated (ETG) electronic paint thickness gages digital type gages are able to read film thickness on both ferrous and non-ferrous metal panel. Digital (ETG) gages may have an accuracy range of 1 percent and include thickness standards for recalibration. The following paint thickness gages are available. Call 1-800-GM-TOOLS for information: