# Assessing Paint Thicknesses - Getting The Most Out Of A Single Layer PTG



## Dave KG (Feb 23, 2006)

The thickness of paint on a car determines how aggressively it can be polished in the quest to remove surface imperfections such as swirl marks or scratches. Assessing what thickness of paint is "healthy" and what is "thin" however is very difficult and requires a lot more effort than simply placing a Paint Thickness Gauge (PTG) on the paint and comparing the reading to a "known" range. The problem lies with the fact that all manufacturers can use different paint systems with varying thicknesses which makes it difficult to pigeon-hole specific readings as healthy, thin or otherwise.

The vast majority of paint systems used on cars today come in three layers: the base coat, the colour coat and the clear coat as shown below.










In these cases you are only ever polishing the clear coat (which is why your pad doesn't turn the colour of the car). Polishing through the clear coat is known as "strike through" - terminal paint damage that can only be repaired with a re-spray and should therefore be avoided at all costs!

The paint thickness you most want to know is that of the clear coat - however, the majority of paint gauges on the market give you a total thickness reading only. On the face of it this may seem like a useless reading - how do you know how much of the (say) 110µm of paint is clear coat? One answer is to use a gauge which gives you multi-layer readings. However these are expensive and generally out of practical reach for most detailers. Another answer is to use your total layer thickness gauge as a detective, and read the information it gives you wisely. A single reading on its own means nothing. Multiple readings across the car allow you to build up a picture and leave you better informed as to the most appropriate course of action for machine polishing.

*Consistency is the Key*

With different manufacturers using different paint systems, a reading which may be thick for say a Mazda may be thin for a Volkswagen. Some cars, for example the Ferrari 355, were hand painted and have readings three or four times thicker than what you would find on a 21st century hatchback. Even the same make and model of car will vary for a variety of reasons. Despite there being no consistency across the paints of different cars, you should see a consistency in the paint thickness of the car that you are working on.

For example, all side panels you would expect to be roughly in the same paint range. Likewise, you would expect consistency in thicknesses across say the bonnet. There will be variations but you would not expect them to be large. Rather than picking up on individual readings the aim is to see how consistent the readings are on the car and if there are any thick or thin regions which stand out which would point to either repair of previous heavy polishing. In both cases you would want to exercise great care when machine polishing.

You can then take this one step further - consistency across a panel. Now if you look at a painted panel, you can generally see an orange peel effect which points to an inconsistency in thickness. The peaks and valleys look to have very different thicknesses but this is actually just an optical illusion - the variance is typically only a few microns and a small percentage of the total thickness. With this in mind you would not expect to see large variances (of order 30, 40, 50µm) across a panel. So you can look out for specific regions that show readings notably different to the panel as a whole, giving you an indication of possible thin or thick spots.

*In Practice - Assessing Before Polishing*

Before using a machine polisher it is important to assess the thickness of paint you have available to you. In this section we will look at how to use a single layer paint thickness gauge to its full potential to give you as much information as possible about the paint thickness available for polishing.

Take the paint measurements in two main stages: general readings across the whole car; detailed readings over individual panels.

_1 - General Readings Across The Car_

The first set of readings to take are across the car as a whole, with a "few" readings per panel (10, 20 or so). Take a note of the readings and at first compare individual panels to each other.

Take the following example of a car with paint thickness readings shown in the diagram below (simplified to one reading per panel for this discussion).










Here we can see that all of the panels have roughly the same readings in the range of 110µm to 130µm. It is generally assumed that readings of this level are "normal" as well, and that the thickness is "healthy". However the numbers really don't give you this information, they simply show that the readings are consistent across the car. This is a good indication that the car has not seen any paintwork repairs and is also a fair indication that there has been no aggressive polishing previously but this latter assumption should not be read as gospel!

Compare this now to the example shown below.










Contrary to the first example which showed similar paint readings, this car shows two clear inconsistencies. Namely the rear passenger side door and the front driver's side door.

The rear passenger door shows a paint reading which is abnormally high compared to the rest of the car. This is a good indication that this panel has seen a paintwork repair at some point as resprayed panels typically have thicker readings. If all of the car gave these readings then this would be an indication of a possible full respray. However you could not say this for sure as the car itself may have just had thicker paint from the factory. The biggest telling factors here are the inconsistencies rather than the actual paint readings.

The front driver's door is showing readings which are abnormally low compared to the rest of the car. This is an indication that this region has been heavily polished or compounded before as there has been notable paint removal. In some instances scratches are removed by wet sanding which, when done without consideration, can leave low regions of paintwork. In the case of all of the car having thin paint readings it is a possible indication of full machine polishing previously. However it could also be that the paint thickness is just naturally thin on the car.

_2 - Detailed Readings_

Having assessed the panels of the car against each other to get a global picture of which may have seen some sort of work in the past, it is now time to look at the panels on their own. It may be tempting to just take a few paint readings and go round the full car in a few minutes. However this will not give you all the information you need to be polishing the car safely.

In this section we look at taking detailed paint readings across the paint on a car. Having spent a lot of money on a paint gauge, you are best to get the most out of it! So spend a good hour or two taking paint readings. It may sound ridiculous but it may just save you from a strike through!

Take the bonnet as an example. Take a paint reading every inch to two inches. An example of some paint readings from a bonnet is shown below.










The consistent reading on the bonnet is between 110µm and 135µm. This is a typical variation across the panel. However we can also see two readings which raise concern - one in the top left and the other bottom middle, both highlighted. One reading is much thicker than the rest of the panel which points to a small region of paint repair or even filler. The other regions shows a thin spot on the panel which points to previous repair by abrasive polishing or wet sanding.

If you were only to take three or four readings per panel, then small regions of concern like the ones above would be missed. This clearly shows the importance of taking many readings across each of the panels. In the same way as you looked for consistency between panels, you should also check for consistency over a single panel as well. Any inconsistencies give you a good indication of vital clues to make your polishing safer.

_3 - Interpreting the Readings_

As discussed above, looking for inconsistencies is the key to getting as full a picture as possible of the paint on a car. Here we look at what the paint readings may be telling you.

In the case of general readings across a car, any panels giving thicker than average readings can point to a region of paint repair. Normally repsrayed panels are thicker owing to the processes used to spray the paint in the bodyshop. You could expect to see readings of in excess of 200µm. You may think that because the paintwork is thicker that you have more clearcoat to play with and thus more aggressive techniques can be used if required. But this is frequently not the case, as all layers of the paint (base, colour, clear) will be thicker. It is best to err on the side of caution on a resprayed panel and treat it with care. Also, expect the paint hardness and polishing style to vary on resprayed panels as the paint system used may be different to the factory painted panels.

If a panel is giving very thick readings or not reading at all then this is an indication of possible filler on the panel. This can trick you into thinking there is a lot of paint to play with when in actual fact you have thin paint over a lot of filler! This is a rare occurrence but something to be very aware of.

For panels that are giving thinner readings than the average on the car, this points to previous machine polishing or sanding. In these cases it is best to err on the side of caution and use only gentle polishing techniques. It may be the case there is still plenty of clearcoat on the panel but it is a risk which is not worth taking - polish lightly and avoid the use of aggressive compounds.

On a single panel with detailed readings, small thick regions can point to smart repairs (which can typically have very different behaving paint), filled chips or scratches or filler. In all of these cases, exercise caution and use only light polishing techniques, building up the aggression by monitoring the paint removal (see below) carefully. Thin spots point to small repairs, for example by wet sanding, which should only be polished gently to avoid making the clearcoat dangergously thin or even striking through. It is important to take plenty of readings as the thin spots can be no bigger than a postage stamp, but once compounded and struck through, the whole panel will need a respray to get the best quality repair.

A key message here - take plenty readings, and look for inconsistencies and use them to build up a picture of the paint in front of you. A single reading, on its own, tells you nothing. But taking multiple readings and analyse them carefully and you can get a lot of useful information about the paintwork in front of you!

_What About Door Shut Readings?_

On many cars you can get a good indication of the thickness of paint without clearcoat (and thus, the thickness of the clearcoat itself) by measuring inside the door shuts. Less clearcoat is used on these areas and this can give a good idea of the clearcoat thickness. However this is not a fail safe technique and in some cases there can be thin colour and base coat used on these regions as well. This can lead to a reading on a door shut of say 60µm while on an exterior panel the clearcoat strikes through at 80µm. Use these readings as another part of the story, another useful piece of information. However it is important not to take the readings here too seriously and definitely not use them as an indication as to how thin you can make the paint on exterior panels.

*In Practice - Assessing During Polishing*

Equally important is the thickness of paint being removed by the polishing process. The clearcoat is essentially a sacrificial layer designed to protect the underlying paint finish but it also adds gloss and gives the final finish as well. The thinner the clearcoat is made, the more its long term integrity can be compromised. Not only that but if the clearcoat is made very thin after one set of polishing, it means that the car cannot be polished months or years down the line when it may be required again. When polishing it is important to achieve sensible correction, the desired level of correction with as little paint removed as possible. To do this, knowing how much paint each process is removing is very important.

It is not possible to use a PTG on a single spot before and after polishing to get an idea of the amount of paint removed as this is subject to large fluctuations for gauge error and the fluctuations of thickness across small regions of paint. For this reason it is best to measure the thickness of the paint over a small area as an average both before and after and then subtract the averages for an approximation of the paint thickness removed.

_Method For Assessing Thickness Removed_

It is all about averages! On the test section, choose a landmark in the paint - this can be something like a stone chip that you will not polish out. You can also create a landmark by using an LED or laser pointer. If using the latter be careful not to stare directly at the beam and also be very careful of stray reflections from mirror surfaces on the car - lasers can cause permanent eyesight damage and should be used sensibly, with care.

Over a region of about 1 square inch around the landmark, measure 10 - 20 paint thickness readings, taking note of each one. Calculate the average of this reading. After a polishing set, repeat this measurement and calculate the average again. Subtract this average from the first one calculated to find the thickness of paint removed.

_Calculating an Average_

To find the average of a set of readings, add all of the readings together and then divide the result by the number readings. As an example, say you have the following five paint thickness readings:

•	121, 119, 117, 124, 120

To find the average, add all the readings together:

•	121+119+117+124+120 = 601

Then divide the result by the number of readings, in this case 5:

•	601 / 5 = 120

_Intepreting the Results_

The key results here are the amount of paint removed by the polishing process. You want to keep this at a minimum while still achieving the desired result.

However, in some cases, it is not possible to achieve full correction and removal of every single deep mark. Use the results of the removed thickness sensibly - if a mark is not removed after the removal of a lot of paint (say 10 - 20µm) then it is prudent to leave the mark. Typically the odd RDS will not be noticed at the end result and it is far safer and better for the finish to leave them rather than removing huge amounts of paint which can compromise the integrity of the clearcoat.

*Summary*

The total thickness of paint can tell you a lot about what is safe and what is not for polishing - both the initial thickness and the amount of paint removed during polishing. When machine polishing it is important that you remove only a safe amount of paintwork. The use of a single layer gauge can give you a lot of useful information to help you make the decision as to what is safe and what isn't. Use the gauge as a detective aid rather than simply reading the numbers off as a reading on its own means nothing. But the readings together allow you to build up a big picture of the car you are working on.

_Acknowledgements: GlynRS2 & robz (Car Plan Diagrams)_


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## Mirror Finish Details (Aug 21, 2008)

Always well worth reading again.

You are a top man Dave.


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## Skodaw (Mar 19, 2006)

Wow Dave,yet another thorough and informative guide, one especially usefull to me as I'm about to become the proud owner of a Paint Depth Guage - and a new rotoary..


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## amiller (Jan 1, 2009)

Cup of tea and a muffin at the ready...

EDIT: Couldn't have said it better myself. Top man.


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## Christian6984 (Dec 20, 2007)

Great help on paint thinkness assessing. If taking paint readings on a waxed car would this give higher results for example if you took paint readings after a wash, but then undertook correction at a later date.


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## Dave KG (Feb 23, 2006)

Christian6984 said:


> Great help on paint thinkness assessing. If taking paint readings on a waxed car would this give higher results for example if you took paint readings after a wash, but then undertook correction at a later date.


Wax layers are submicron thickness, so they wouldn't register on the gauge


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## caledonia (Sep 13, 2008)

I am sure this review will benefit a good few members Dave.
I hope this highlight the benefits of have the necessary equipment and also save a good few member the heart ache of a strike though.
Having been in you company a good few times now. I have witnessed the care and attention you put into paint reading. So in turn cutting down possible mishaps., and conserving the very important clear coat.

Great review as alway and thanks once more.
Gordon.


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## PaulN (Jan 17, 2008)

Great Read there Dave.

Pretty valid to me as im Blowing the cob webbs off the G220 later!

Cheers

PaulN


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## spitfire (Feb 10, 2007)

Hi Dave, informative write up as usual. :thumb:

I also seem to remember a point you made at one of the meets about false readings when checking during polishing sets due to heat build up. I deduced from that, that readings can be different on a hot sun drenched panel as against a reading taken in shade. Would this be a correct assumption? If so it's a point to bare in mind for those polishing outdoors in summer I suppose?


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## Guest (Jun 12, 2009)

Excellent article :thumb:

Would you take multiple readings on the same spot to reduce the gauge error when profiling a panel?


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## graemeforsyth (Jul 11, 2008)

Yet another very well written "print and keep" piece from you Dave. Many thanks.

Regards Graeme


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## TimG (Apr 2, 2008)

Good read in which serves as a reminder for folks like me; forgetful LOL


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## Dave KG (Feb 23, 2006)

spitfire said:


> Hi Dave, informative write up as usual. :thumb:
> 
> I also seem to remember a point you made at one of the meets about false readings when checking during polishing sets due to heat build up. I deduced from that, that readings can be different on a hot sun drenched panel as against a reading taken in shade. Would this be a correct assumption? If so it's a point to bare in mind for those polishing outdoors in summer I suppose?


Yes, spot on - avoid taking readings on hot panels as you can get artificially high readings, and only on the hot panels which will play havoc with the assessment of your work.



Phisp said:


> Excellent article :thumb:
> 
> Would you take multiple readings on the same spot to reduce the gauge error when profiling a panel?


I do that for test spots, but tend to take readings every cm or two and although its only one reading, by being close together it helps reduce error in the assessment if not the actual number as you are essentially double and triple checking a same general area


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## Hair Bear (Nov 4, 2007)

This needs to be a sticky


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## Bigpikle (May 21, 2007)

missed this first time around Dave - but great guide.

Is it worth a reminder to calibrate your gauge EVERY time you use it, and to remind people of the error % in all gauges.

It seems that there is so much half knowledge about the single thickness and multi-layer gauges and spending time with Jon learning about his collection of gauges has been very helpful. Without good knowledge and experience of paint and painting as well, it can be hard to interpret fully the information they provide often. Its an area I'd like to learn much more about.


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## Dave KG (Feb 23, 2006)

Bigpikle said:


> missed this first time around Dave - but great guide.
> 
> Is it worth a reminder to calibrate your gauge EVERY time you use it, and to remind people of the error % in all gauges.
> 
> It seems that there is so much half knowledge about the single thickness and multi-layer gauges and spending time with Jon learning about his collection of gauges has been very helpful. Without good knowledge and experience of paint and painting as well, it can be hard to interpret fully the information they provide often. Its an area I'd like to learn much more about.


Its the case with any measurement device, you have to understand what it is measuring, how accurately it is measuring it (no point quoting 50 decimal places if the error is plus or minus a single unit ), and most importantly, what the measurement is telling you... I often see measurements being made with no sanity check and you're on a hiding to nothing in that case.

The different paint systems mean you do noeed to be careful how measurements are interpreted, especially mutlilayer gauges which should never be used as the gospel they are sometimes said to be - paint layer interfaces are typically not very well defined, you can see that when you strike through that it starts with very small patches 

Its a wide ranging subject and hopefully throughout the course of the year, we'll all be able to contribute to enhancing the knowledge and the use of gauges.


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## Bigpikle (May 21, 2007)

absolutely true, and dont even start thinking about older cars and what might be lurking there....

we put all manner of gauges on the MG and came up with a very puzzling set of readings that were frankly impossible to interpret unless you knew how the car was painted? Some panels had bare metal resprays, but maybe others hadnt? Is the top layer reading really clear or is it actually the respray and underneath is a different colour. Even a 300+ micron reading was frankly useless without more information.

Then we put it on my CF framed road bike - it started to get really fun at that point


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## Epoch (Jul 30, 2006)

Bigpikle said:


> absolutely true, and dont even start thinking about older cars and what might be lurking there....
> 
> we put all manner of gauges on the MG and came up with a very puzzling set of readings that were frankly impossible to interpret unless you knew how the car was painted? Some panels had bare metal resprays, but maybe others hadnt? Is the top layer reading really clear or is it actually the respray and underneath is a different colour. Even a 300+ micron reading was frankly useless without more information.
> 
> Then we put it on my CF framed road bike - it started to get really fun at that point


I did say let me scratch it with one of Steve's PIG gauges (destructive testing is the only way  )


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## Bigpikle (May 21, 2007)

Epoch said:


> I did say let me scratch it with one of Steve's PIG gauges (destructive testing is the only way  )


has the swelling gone down yet?


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## Epoch (Jul 30, 2006)

Bigpikle said:


> has the swelling gone down yet?


:wave:


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## spitfire (Feb 10, 2007)

Interesting reading chaps. The expensive PTGs are obviously more accurate but maybe not giving a ********** answer to over all view of the cars paint layers. I hear what your saying. That being the case, and with only the cheaper overall thickness gauge at my disposal. I'm wondering just how much use they are. All that can really be said is that they'll give an *idea *of how much paint your removing. The rest, without knowing the cars history, is mere conjecture surely.


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## Epoch (Jul 30, 2006)

spitfire said:


> Interesting reading chaps. The expensive PTGs are obviously more accurate but maybe not giving a ********** answer to over all view of the cars paint layers. I hear what your saying. That being the case, and with only the cheaper overall thickness gauge at my disposal. I'm wondering just how much use they are. All that can really be said is that they'll give an *idea *of how much paint your removing. The rest, without knowing the cars history, is mere conjecture surely.


I like to think of them as a sense, they give you something you can't touch, see, smell or hear. It's what your brain does with th information as to how effective the readings are.

Damon's car was a classic, some panels were bear metal respray (he has pictures) some were very thick etc. The gauges would have rung alarm bells that no other sense (other than hunch, the most relied on of the senses) would have picked up. The car has a full respray so looks identical all over. Approaching this car post gauge readings would have swayed me one way to how to approach the polishing.

New cars can be as bad as older cars for hidden unexpected dangers

The rolls we did on Sat has had a hard life and had had 40 plus microns removed off some areas previously and it was a 2008 car, why can i say this

1. After speaking to the manufactures paint division before we went i knew what they are painted with (tolerances which the machines spray etc) I knew what finishing processes they go through and the accepted removal rates. so had an idea how much paint should have been on the car.

2. The total gauge indicated huge inconsistencies in thickness on metal panels (something the manufacturer info confirmed was post manufacture).

3. The composite gauge indicated again varying thicknesses on the wings and bumpers

4. The composite gauge showed potential that the top layer was very thin (again would be true if the surface had been polished)

5. And the final piece of the puzzle it looked fubard and felt like it needed a good claying. A few areas also looked to cause some concern.

This built up a picture that allowed us to as "safely as possible" work on the car to satisfy the owners requirement.

While i forgo the gauges weren't the only thing we used to work on this vehicle they did significantly enhance the picture.

A gauge is a valuable tool as long as you understand what it tells you.

Cheap ones work and are much better than nothing

The expensive metal ones are more accurate (1% instead of 2% accurate so largely unnecessary unless some of the other features are of benefit)

The very expensive composite ones are useless unless you know how they work, how to use them and what you are seeing.

So I agree, they give you an Idea


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## spitfire (Feb 10, 2007)

Epoch said:


> I like to think of them as a sense, they give you something you can't touch, see, smell or hear. It's what your brain does with th information as to how effective the readings are.
> 
> Damon's car was a classic, some panels were bear metal respray (he has pictures) some were very thick etc. The gauges would have rung alarm bells that no other sense (other than hunch, the most relied on of the senses) would have picked up. The car has a full respray so looks identical all over. Approaching this car post gauge readings would have swayed me one way to how to approach the polishing.
> 
> ...


The one thing that DW can't give a newbie to machine polishing is experience. I certainly wish there was an expert detailer round my way. Someone to show me first hand how to become better at the art of machine polishing and detailing in general. In truth, I'm a bit envious of Big P. Sadly, I have to learn by my mistakes.(Hopefully on spare panels):lol: It's not the first time a useful post like this one has appeared after I've made a boo boo:lol:However I have a background in the motor trade and have that to my advantage. A lot of people on here are not so fortunate, and are starting with a severe lack of knowledge in car paint prep. Thanks to people like you, Dave, Steve, Clark and many others, that lonesome trail is not so steep. Posts like this are invaluable to the masses. Keep up the good work guys.:thumb:


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## Epoch (Jul 30, 2006)

spitfire said:


> The one thing that DW can't give a newbie to machine polishing is experience. I certainly wish there was an expert detailer round my way. Someone to show me first hand how to become better at the art of machine polishing and detailing in general. In truth, I'm a bit envious of Big P. Sadly, I have to learn by my mistakes.(Hopefully on spare panels):lol: It's not the first time a useful post like this one has appeared after I've made a boo boo:lol:However I have a background in the motor trade and have that to my advantage. A lot of people on here are not so fortunate, and are starting with a severe lack of knowledge in car paint prep. Thanks to people like you, Dave, Steve, Clarke and many others, that lonesome trail is not so steep. Posts like this are invaluable to the masses. Keep up the good work guys.:thumb:


Thanks

I still have a lot to learn about paint, so you have a head start there :thumb:


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## murproject (Mar 10, 2009)

Great article:thumb:

Which gauges do you prefer ?:speechles


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## RandomlySet (Jul 10, 2007)

cracking post mate

:thumb:


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