purpose of a Paint gauge

what exactly is the purpose of the paint gauge? I know it is to measure how thick the paint is and to find out whether or not a panel has been resprayed. But once you know it's been resprayed then what?

 

it is also a way to see if there is enough paint to polish... is not fool proof as the paint may be thick and the clear really thin but it helps you gauge that... for example if the paint is ready 50um changes are good you are very near to burning through the clear.... so its just another way to make sure you dont burn through the clear

 

ya i was wondering what you base the reading on. because even between different manufacturers im sure each layer of clear coat must be a different thicknesses right?

but for a respray definately there will be a difference in each coat.

 

Most factory paint jobs range from 3.5 to 5.5 mils (87 to 137 microns) (this will vary between manufacturers) for this reason, it’s also recommended that paint be measured when buffing since you can’t see how much paint is being removed during the buffing process. If an insufficient thickness of clear remains after polishing premature failures such as fading or delaminating may occur as the colour coat won't be protected from ultra violet (UV) radiation and a warranty issue may develop in the future. Such risks can be alleviated by monitoring and minimizing the amount of top coat removed, which is best done through quick and easy measurements with an electronic paint gauge, and maintaining a record on a paint thickness template.

German and Japanese cars: 4-5 Mils (1 Mil=1000th of an inch)
With the exception of:
Lexus-White and Tan: 5-7 Mils, Lexus- Black and Silver: 4.5-6 Mils
BMW: Silver: 6-7 Mils
FORD: Approx. 5.5-7 Mils
GM Cars and Trucks: Approx. 4.5-6.0 Mils, Jimmy’s and Blazers: approx 7 Mils
Chrysler: Approx. 5.5-7 Mils (Note Sebring hoods are fibreglass)
Dodge Cars: Similar to Chrysler
Dodge Trucks and newer Ram Pick-ups: Cab approx. Mils and the boxes approx 9 Mils
Jaguar: approx. 6-7.5 Mils (but they tend to vary)
Jeep Grand Cherokee: These read approx. 6-8 Mils, Wranglers: approx. 4.5-5.5 Mils

Note- the above are provided for information only

It’s interesting to note that painters must now demonstrate proficiency with an electronic paint thickness gauge in order to become certified to perform refinish warranty work for General Motors Corp. (GM) vehicles.


Paint Thickness Gauge - Paint Film Thickness - Detail University - Auto Detailing Forum

 

I also use my PTG to measure how quick the paint is coming off. Even after I ensure the paint can be polished I measure, do a single pass and measure again.

 

If your not using a rotary is a PTG still an essential tool to have?

 

I use a paint gauge daily and look for 2 things:

A: If the paint is too thin, I know it has been cut/compounded to a dangerous level and will not stand any more heavy compounding.
B: If the paint is too thick, I know it is a re-paint and will probably behave differently to compounding and polishing. This is especially true of German cars with ceramic or hardened clear coats. Repainting can't duplicate that process so the paint will generally be a lot softer in the repainted panel.

The gauges are good insurance for a detailer. Most run from $400 to $1,000 for a top of the line.

Toto

 

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.

 

PS:

Re: how much clearcoat is acceptable to leave on. Leave as much as you can. It may be tempting with the guage to remove a lot more clearcoat to get the last off RDS out. But be very aware that the clearcoat relies on its thickness to adhere to the underlying paint. If its too thin, it cannot adhere strongly an you get a flaking effect at which point its respray time.

 

Do many people on here use these? In the years I've been detailing, I haven't ever come across someone who uses them, and never even heard of them until I saw Paul's video online a while back..

 

I use them

 

so why are resprayed panels so much thicker than stock? What paint layer makes it thicker? or does it just vary depending on who sprayed the car?

 

Cars painted by hand typically have thicker paint than those sprayed by robots in the factory - just humans are not as able to get a thin even layer as a programmed machine.

Hand painted cars, like the F355, are thick out of the factory: 300um+ not unknown. My robot sprayed S60 is around 120um.

The thickness does depend on who sprays the car, and also how many layers o base, colour and clear coat are applied. I have seen a car that had 17 layers of CC applied, gave a very deep gloss to the paint job, and the paint was 500um+!

 

wow you are like the detailing genius. thanks for the help

 

Bear in mind that a clear coat has a thickness of 1.5 – 2.0 Mils, removing more that 0.3 mil
(0.0003-inch or 14 µ) of clear coat will cause premature paint film failure. As a point of reference two sheets of Saran wrap placed on top of each other measure 1.5Mil (0.0015") a surface scratch that will `catch' your fingernail is approximately 0.003-inch - 14 microns deep and will usually require wet sanding and refinishing.

There comes a point when you must judge wither removing a scratch will compromise the clear coat and if so you’ll have to ‘live’ with the imperfection. Check the paint with a PTG to see how much paint there is available before attempting repairs Clear coat that is too thin looses its ability to adhere effectively (delaminating) to the underlying paint layer and will flake off


For our British readers-:angel: Comparison between millimetres and microns; a Mil is equal to 1/1000 of an inch which can also be represented as 0.001- inches or 0.0254mm (mm = millimetre). A micron is equal to 1/1,000 of an mm or 1/25,000 - inch or 0.00004- inches. There are approximately 25 microns to one mill (0.001- inch) Microns to mils: divide by 25.4

Sorry Dave couldn't resist as I know you teach physics

 

copied and pasted to a note pad for my reference in future.