Question about microns of paint

[the other pc]

Originally Posted by Setec Astronomy

Are these ultrasonic, rather than eddy current? ....

Yes, the PosiTector 200 and 100 series Coating Thickness Gages are ultrasonic.

Originally Posted by MuttGrunt

...A Defelsko representative told me that while it can still be used, one issue likely to come up is you won't get readings aside from the clear-coat and everything else. ...

I believe what he told you is an oversimplification.

Originally Posted by MuttGrunt

.... In short: you'll get a reading of the clear-coat, but the base coat, primer-sealer, primer-surfacer, and E-coat/self-etching primer will likely be grouped together as there isn't a big enough difference in the density in those layers for the tool to tell a difference in them.

This will help to show you that even though it seemed to read differently, you're clear-coat is, like others pointed out, still going to be the thickest layer. Everything other than the clear was just lumped together....

This may be the core of that oversimplification.

It is certainly true that an ultrasonic gage may or may not be able to differentiate between layers. It depends on how much of an “echo” is returned from the material boundary and the sensitivity of the detection method.

The quality of the echo is dependent on the how closely the materials are matched in “characteristic impedance,” which is a function of a material’s density and the speed of wave travel through it. (When the material is air, it may be called “acoustic impedance.”)

The more closely two materials’ impedances match, the less echo, because sound propagates through them in the same way. The greater the difference between their impedances, the larger the echo.

Since different layers of an automotive coating system can have rather similar chemical/physical composition there may sometimes be little difference between two layers’ impedances. So there may be little echo. But it’s really dependent on the specific combination.

There’s no reason an ultrasonic won’t work on metal substrates. They have their own characteristic impedance like any other material. In fact, ultrasonic measurement is often used for checking pipe wall thickness. But I don’t doubt that magnetic and eddy current techniques are more accurate on metal substrates.

Any sort of wave propagation has some equivalent to characteristic impedance. In the field radio wave transmission it’s also called characteristic impedance. In the field of optics and light transmission, it’s called index of refraction.

In open space, the radio wave equivalent of what the PosiTector does is called RADAR. For enclosed cables, the radio wave equivalent is called a Time Domain Reflectometer. The equivalent machine for fiber optic cables is also called a Time Domain Reflectometer. The equivalent in water is called SONAR.



pc.

[efnfast]

Originally Posted by Street5927

I would check the manual to see what the readings are, however, I think it is unusual to have thicker clear than base.

My friend told me that the first reading on it is the clear, which would make sense since that's the first layer. (I just borrowed it from him for a few minutes, he's the expert, not me, heh, just wanted to see what others were getting for various readings)

Maybe somebody whose used a Positector 200 religiously could chime in because I'm starting to get really confused, heh.

Again, this isn't a production car (entire car is fiberglass), or a production paint job - hand sprayed with 4coats of clear FWIW.

[Showroom Shine]

This is some very Good info on the gauges and paint mils and microns. Real techy stuff. But never the less useful. So how many of you PRO'S are using the guage?Do you have to have it for every car?

[MuttGrunt]

Showroomshine - We use one on any car that will have paint correction performed. It's always good to get an idea of where there may be any problem areas.


Thank you to The Other PC. Very good in-depth explanation. The representative didn't seems to over-simplify, rather I'm trying to paraphrase for the sake of a quicker post. The information you posted will be of a huge help to anyone who would like in-depth info onto how these ultrasonic meters work.

To others: Your clear-coat is the thickest layer. It sprays on thicker, doesn't easily lay flat, and gives off seemingly stronger VOC's compared to spraying base-coat. Just because your Coating thickness meter groups the base-coat+E-coat together (factory) or base-coat+sealer+primer-surfacer+self-etching primer together (aftermarket respray) doesn't mean the clear is thinner than the base. As The Other PC pointed out, it's hard for the PTG to differentiate in the layers under the clear because of sensitivity settings from the factory.

[efnfast]

Originally Posted by MuttGrunt

To others: Your clear-coat is the thickest layer. It sprays on thicker, doesn't easily lay flat, and gives off seemingly stronger VOC's compared to spraying base-coat. Just because your Coating thickness meter groups the base-coat+E-coat together (factory) or base-coat+sealer+primer-surfacer+self-etching primer together (aftermarket respray) doesn't mean the clear is thinner than the base. As The Other PC pointed out, it's hard for the PTG to differentiate in the layers under the clear because of sensitivity settings from the factory.

So from what I wrote above, would you say I have around 40microns of clear, or more?

[MuttGrunt]

Originally Posted by efnfast

So from what I wrote above, would you say I have around 40microns of clear, or more?

I would believe this to be correct, and doesn't seem unusually thick nor thin. Some paints will have more clear / thicker clear than others depending on many factors of course, but averaging 40 microns for your clear seems about right.