The minimum paint thickness is an indication that there is no paint left to polish and should be avoided as much as possible. The downside is that this thickness is difficult to monitor or measure. Going passed this minimum thickness will result in strike-through of burn-through.
Why have a minimum paint thickness
When polishing, you remove a very fine layer of the surface. The amount you remove should be equal to the depth of the deepest surface imperfection you are trying to remove. The downside is that you can not re-apply the paint you remove. So every time you polish the surface, you remove a thing layer. Eventually you’ll run out of paint to work with.
The thickness of your paint is 120 micrometer. You have been able to measure this with a paint thickness gauge.
The primer has a thickness of 20 micrometer.
The colored coat has a thickness of 60 micrometer.
The clear coat has a thickness of 40 micrometer.
Every time you polish the surface you will remove a very fine layer of the surface. To remove the imperfection that made you decide to polish, you need to remove the amount of the depth of the deepest imperfection. For the sake of this example, lets say this is 10 micrometer each time you polish.
The clear coat has a thickness of 60 micrometer, which means you can polish up to a maximum of 6 times until you run out of clear coat. Once the clear coat is polished off, it is considered a strike-through and the colored coat is exposed.
If you continue to polish you will start to polish away the colored coat. As we can see above, this has a thickness of 80 micrometer. You will have a maximum of 8 times polishing until you will run out of colored coat. If the colored coat is removed it is considered a burn-through and the primer is exposed.
In total you would have (theoretically) a total of 12 times polishing until both the clear coat and the colored coat is gone. If you polish the vehicle every 6 months, you’ll run out of paint withing 6 years.
If you could have a minimum paint thickness, you could say how much paint you have left to work with.
Impossible to name 1 minimum paint thickness
It would be best if there was 1 number you needed to remember as a minimum paint thickness, but this is impossible to do. Paint is very different between panel, vehicle and brand. If the colored coat has a thickness of 80 micrometer in one part, it could be 20 micrometer less only 1 next to it. The same goes for the clear coat. It could be 50 micrometer at one spot, and 30 micrometer only an to the left. It would be impossible to give 1 number that would work on all vehicles, surfaces or brands. It is even impossible to give 1 number for a panel, or as much as an area of 10 square . This is one of the difficulties that detailers face: how long will it be safe to polish?
A general rule of thumb
If you have the proper equipment you can measure the paint thickness. With a more expensive model you can even see the thickness of the primer, colored coat and the clear coat. This will give you a rough idea to work with. The general advice is to measure the paint roughly every 10 or 15 in a grid pattern on a panel. Write these down and calculate the average of the thickness of the clear coat. It should always be the aim to never polish away the clear coat. Take the lowest number of thickness and the average, and add the difference between these two to the average. This would be your guideline.
These are the measurements on a panel: (the number shows the thickness of the clear coat)
Spot 1: 40
Spot 2: 41
Spot 3: 56
Spot 4: 38
Spot 5: 44
Spot 6: 50
Spot 7: 38
Spot 8: 46
Spot 9: 39
Spot 10: 41
Spot 11: 42
The average is: 43 micron.
The lowest number is: 38 micron.
The difference between the lowest number and the average is 5 micron.
This tells us that the average difference between one spot and another spot is 5 micron. This number would be your minimum clear coat thickness to polish. If you would measure in the future and see that the thickness of the clear coat is around 5 micron, it is very likely that it would be (close to) 0 micron on another spot on the same panel. This makes it very dangerous to continue polishing. It would be best to not polish this panel.
Do not use a minimum paint thickness
It would be best to NOT use a minimum paint thickness. The danger is that you will start to work with averages, which is not accurate. The best method is to measure the thickness carefully and then make a judgment call. Working with average still leaves room for error. In this case this would lead to a respray, which is a costly mistake to rectify.