Paint Thickness Gauge

[imported_Indy YZF]

I have one of those Mar-Hyde pen shaped magnetic paint thickness gauges. It shows that IVe got between 6 and 7 mils of paint on my various BMW surfaces, so whether its accurate or not, I think the results are at least reasonable.



But, about the accuracy. I realize that people dont feel they have much more value than being able to tell which panels are steel and which are bondo!



However, it has a calibration screw in the head of it. The gauge registers '0' mils when testing the steel on the top of my stove with nothing between the surface and the gauge. I dont see why, calibrating it using a piece of brass shim stock of known thickness over this steel, I cant 'force' it to give accurate readings. The scale only has 1 mil calibrations. But, for people only looking for a general reading, ie, to discern the difference between 4 to 5 mils where you wouldnt want to use anything stronger than perhaps a finishing polish, and 6 to 7 mils where you might be more comfortable using something a little more aggressive, or 8 mils where you might be comfortable with a moderate rubbing compound or even some light color sanding, I dont see why this couldnt work. Obviously its not going to be a good enough tool if youre color sanding and you want to make sure to not remove more than .2 mils leaving yourself up to two more tenths for compounding away the scratches.



But for the more general, less granular readings that it takes, why cant it work? Once youve calibrated it, it seems to be more a question of resolution than it does of accuracy.

 

[az57chevy]

Do you get the same reading consistently ? I think sometimes the mags are guides but not accurate measures

 

[imported_Indy YZF]

Do you get the same reading consistently ? I think sometimes the mags are guides but not accurate measures

Yes... if you use the same technique, ie, hold it gently between thumb and forefinger on the end, making sure the magnetic is flush on the surface and flat, and slowly lift the unit, it will deliver the same result repeatedly. Obviously, if somebody holds it on an angle one time and straight on the next, it wont deliver the same result.



I dont have the shim stock yet, as I just ordered it.. .005" in brass. But Ive been playing with it using my steel top stove as the base, and sure enough, it constantly records a ZEro thickness when checking that. What has me concerned, though, is that I tried using a sheet of paper, and I dont like the result. I measured a piece of regular lined paper at .003" using my digitial calipurs. But when I placed that on the stove top, my gauge showed a measurement of 6 mils, not the 3 mils I was expecting. Thats when I decided to order the brass stock like it says in the instructions. Maybe the paper, which really isnt attached to the surface and maybe has some air trapped, cant give a good reading. If the unit measures a correct 0 thickness when measuring the stove top directly, I dont understand why any other measurement it takes would be off. When measuring the bare stove top, it lets go at precisely Zero. We'll see what the brass stock of 5 mils does when resting on the stove top.

 

[the other pc]

... But Ive been playing with it using my steel top stove as the base, and sure enough, it constantly records a ZEro thickness when checking that...

In the measurement world that’s called offset. It’s only one part of “accuracy.�

... If the unit measures a correct 0 thickness when measuring the stove top directly, I dont understand why any other measurement it takes would be off. When measuring the bare stove top, it lets go at precisely Zero. ....

Because other pieces of the “accuracy� equation, like slope and linearity, don’t show up at zero. (They’re the important pieces, and the harder ones to get right.)

...We'll see what the brass stock of 5 mils does when resting on the stove top.

If you think paper was bad for not being “attached to the surface,� wait till you try metal.







PC.

 

[imported_Indy YZF]

If you think paper was bad for not being “attached to the surface,� wait till you try metal.

PC.

Then what do you suggest should be used to calibrate? Im only using brass shim stock because thats what they say to calibrate with.. But if its as lousy as paper, then I dont know What to use to calibrate.

 

[the other pc]

Real thickness gauges come with sample standards, metal cards with layers of known thickness coated onto them. (They can also be purchased separately.)



You can make your own since you have a caliper to compare with. What’s the resolution of your caliper? Since your magnetic gauge only resolves to one mil you can’t really expect precision any better than that. If your caliper also only goes to one mil then you’ll get even less precision overall. (You can rarely get one mil repeatability when using a caliper anyway.)



You can use shim stock but you’ll need to bond it permanently to a ferromagnetic base to avoid the air gap. You’ll be adding a glue gap but it will be fixed and repeatable. The trouble with the air gap with loose sheet stock is that it changes any time the stock moves at all. Alternatively, you could just smear on a thick coat of epoxy or paint or something and lap it flat. Make a couple of them with different thicknesses



Measure the base thickness before and after gluing down the shims and subtract the difference to give you the layers’ thicknesses. Do that for multiple thicknesses and draw a plot of readings verses the measured “standards� to draw a correction graph.





PC.

 

[imported_Indy YZF]

Real thickness gauges come with sample standards, metal cards with layers of known thickness coated onto them. (They can also be purchased separately.)



You can make your own since you have a caliper to compare with. What’s the resolution of your caliper? Since your magnetic gauge only resolves to one mil you can’t really expect precision any better than that. If your caliper also only goes to one mil then you’ll get even less precision overall. (You can rarely get one mil repeatability when using a caliper anyway.)



You can use shim stock but you’ll need to bond it permanently to a ferromagnetic base to avoid the air gap. You’ll be adding a glue gap but it will be fixed and repeatable. The trouble with the air gap with loose sheet stock is that it changes any time the stock moves at all. Alternatively, you could just smear on a thick coat of epoxy or paint or something and lap it flat. Make a couple of them with different thicknesses



Measure the base thickness before and after gluing down the shims and subtract the difference to give you the layers’ thicknesses. Do that for multiple thicknesses and draw a plot of readings verses the measured “standards� to draw a correction graph.





PC.

Excellent advice.. You sound like you know a thing or two about thickness gauges! .. My digital calipur, btw, resolves to 3 decimal places so that should be good. as I said, I realize the best Ill achieve is to the nearest mil due to the gauge's resolution. Im mainly interested in making sure Im not down below 6 mils if I plan to try something courser than a light to moderate polish known not to remove much if any clearcoat.. Something like Intesive Polish, or Sonus 2, or Finesse-It. Anything more abrasive than that and Id like to be reading OVER 6 mils on an accurage guage. Although, Meguiars has a couple of very interesting threads where they did measure paint removal with various methods and products.. Turns out that with a Porter Cable and a polishing pad, even #83 removed hardly any paint!