To do what?
It is common to have to correct the varnish of used vehicles that are several years old and sometimes several hundred thousand kilometers on the counter.
Thus, it is likely that these vehicles have already undergone sometimes severe polishing in their lifetime, thus considerably reducing the thicknesses of varnish present. 
 
Also, on moderate to major defects (deep scratches), it is often necessary to "wet sand" the area, an operation which requires removing a large quantity of varnish.
 
For these various reasons, it is strongly recommended to use a PTG (Paint Thickness Gauge) which allows us to measure the thicknesses of paints in order to check whether we can undertake a large polishing work on the area or not.

Step n°1: In which situation(s)? 
- Before any "polished-gloss" in order to remove the orange peel on a vehicle, it is essential to check the thicknesses of varnish.

- Before "wet-sandering" (sand locally dry or wet) a deep scratch.
- Before polishing with aggressive approaches (compound, microfiber pads or woolpads) a vehicle that is several years old.
- Know the painting history of the car (original elements, repainted).