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Design for a Homebrew Paint Thickness Tester
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m3vuv:
what about a banana gauge?,no electronics to go wrong too
Kleinstein:
The capacitive way should be relatively simple to build, but the big unknown is the paint properties, that can change even with color.
There is no need to have a direct contacts to the metal, just a large area at the outside is enough and this can also shield the sensitive part. It would be at least useful for relative measurements, so to see if the paint thickness is uniform, e.g. to find hidden repaired areas.
The extreme version would use the painted area as the end of a cavity resonator and could in theory even measure the Er of the paint too.


The Ultrasonic way has a similar problem with the paint properties, though likely not as bad with different colors.#

The magnetic way sound quite plausible and possible, as essentially all colors of paint are magnetically not active. One way would be to to measure the inductance with a small U shaped core at a relatively low frequency, so that eddy currents (especially in the steel of the car) are not a big factor.  The other option would be to use a rather high frequency (e.g. some 13 MHz) and include the eddy currents. So the steel would be mainly seen as a conductive area, not as a high permeability.
StuartA:
I've had some amazingly goods results from a modern, miniature 220uH inductor (one of the ones that looks just like a resistor).
Placing it on its side on plain steel, the measured inductance goes up to nearly 260uH, then sliding in pieces of 200um thick plastic sheet between the metal and inductor, the measured L falls back down again, useful out to at least 600um. It's quite insensitive to the presence of my hand. I'd say it probably gives me a resolution of ~10um.

The reason I want a gauge is for use in machine-polishing a 20 year old car, which has lots of swirl marks. You really do not want to polish through the clear-coat. You can only measure the clear-coat thickness using infrared, but those instruments are very, very expensive. So, I'm just probably looking at just setting myself an absolute limit of not polishing off any more than maybe 50um (and ideally, less), which should mean I'm still in the clear-coat.
Kleinstein:
For just a relative measurement with clear paint, the capacitive way should also work. The clear paint should not vary that much in Er as it may happen with pigments. Measuring the capacitance change in the pF range is not that difficult.

For inductor way (low frequency), I would consider a U shaped core,  like some ferrite cores used in some common mode chokes with one part removed. The test would be using kind of a variable air gap inductor. One could also try an old audio tape or even floppy head.
m3vuv:
I used to work in ndt on cross country gas pipelines,one of my tasks was to measure the coating thickness on the pipes,we had ultrasonic testers but mostly used a banana gaugesimple to use with nothing to go wrong.
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