My thoughts on the paint gauge... Afraid I don't do mils, as I'm British, so my numbers are gonna be metric, in microns (um).
First of all - do not rely on them as 100% accurate measurements of the thickness of the paint, because this they are not. What they are are guides. Guides to the approximate thicknesses of paint in a region on the panel, allowing you to determine whether or not it is safe to polish or sand the area with certain levels of abrasive polish.
So, faced with a car in front of me, what do I do with my paint gauge? Well, first of all, I loose any preconception of a paint thickness dependant on the vehicle type. You simply cannot judge this way, its the same reasons as you cannot assume that the paintwork will be rock solid because its an Audi, or butter soft because it is a Honda. Every car is different, and has a different history - it may have been polished before, it may have seen the bodyshop before and been painted - who knows?! So we are starting out with guesswork here.
I have in my head the following generic thickness ranges:
<70um: very thin, avoid abrasives if possible
<90um: thin, take care
<100um: quite thin, take care with aggressive polishes
<150um: normal
<200um: thick but not unknown, suspect possible repair so assess paint carefully
>200um: likely repair or hand painted car - paint may respond differently to OEM, be prepared for this
So that is my first guesstimate.
Next up, open a door, measure the thickness on the inside of the door - less clearcoat is used in here, so this can give you an idea of the baseline level of the clearcoat. Say you measure 50um, and on the bonnet you measure 110um - you could guesstimate that there exists ~60um of clearcoat on the bonnet. But again, I stress this is a guesstimate.
The use of a paint gauge is not an accurate hard and fast technique - its a case of using tricks and data presented to build up a picture of the paint and allow you to make more educated guesses of what polishes and products are suitable and what aren't. ITs another tool to aid and another safety net, but its not fool proof.
Measuring paint thickness removed is very hard to do to a high degree of accuracy. The paint thickness caried greatly over very small areas, you have to be very confident you put the gauge down in exactly the same spot. Better is to take say 20 reading around a square inch spot and average them, and then after repeat this and take the difference of the averages. This is more accurate. However, unless the paint is quite soft and you are using medium or more abrasive polishes, the amount removed is actually quite small. And every gauge has a measurement error associated with it, typicall +/-2um. So if the thickness removed is 4um (quite typical), you cannot accurately measure this, as it is inside the wroking error of your gauge.
So in summary - treat the gauge as something to build a picture and give you more data to analyse, but not as a be all and end all of answers. Its a guesstimate tool, rather than an accurate instruction of how what and what is not acceptable to use on paintwork.
