The presence of the gauge does not mean that now you can easily distinguish a broken car on which did not participate in accident. Any device requires proper handling, and also a presence of experience working with the device. Consider the instructions for use of the gauge, as well as features that should draw the attention of first-time users.
To the instrument determined the paint thickness (LCP) with minimum error it needs to be calibrated. For this purpose, with the device (depending on model), there is one or two washers (metallic, non-metallic), and plastic film. Calibration instructions can vary depending on the model, but the principle is similar.
If the body of the car is made up of metallic and non-metallic parts (eg aluminum), it is not necessary to calibrate the instrument for each surface. If you calibrate the thickness piece of iron on the puck, then the measurements on non-metal surfaces testimony will be simply with a little more accuracy.