Paint Correction - What You Should Be Considering!

Machine polishing... Think detailing and this process is what comes top of many people's minds. Its a key aspect of many exterior details, and its function is to get the paintwork looking as good as possible to get the benefits out of any glazes and waxes that are applied after it.

Much is said about the achieved correction on a detail, and the desire is often said to be to get every mark out, ie achieve 100% correction... But what exactly does this mean for your paintwork? In this thread I am taking a brief tour of the paint correction process, how to assess the removal rates and what they can mean to your car's paintwork.


Paint Thickness & Paint Removal

Typically automotive paintwork consists of either two or three "coats" as shown:



The region of paint that you polish is the top-most coat and full removal of this coat will be highly detrimental to the paint finish... If you "strike-through" the clearcoat on your car you have fully removed the protective lacquer coat of the finish:



and this results in a complete loss of gloss to the finish...



The only cure for this is to respray the panel - an expense and also means that the paint on the panel concerned is no longer original and if you have a car that is a future classic, twenty years old, the sudden loss of originality could be soul destroying!

But does this mean that we can polish away large amount of the clearcoat, still leaving a small amount and this be perfectly safe? As I will address in this thread, this kind of process is not always advisable for many reasons - ranging from the thickness of clearcoat having a bearing on its performance to the fact that an everyday car may well need to be polished in the future!



Measuring & Assessing Paint Thickness & Removal

A key tool in the professional detailer's armoury is the Paint Thickness Gauge (PTG). This is a device which allows the measurement of the thickness of paint on a car. Most gauges, and certainly all in the price range easily accessible to the hobbyist detailer, give a reading only of the total thickness of paint on the panel and not an individual break down of the layers. However, this piece of information is still vitally important providing it is read and assessed in the correct manner.


How To Use a PTG

For greater detail on use of a PTG , refer to this thread: http://www.detailingbliss.com/forum/f37/assessing-paint-thicknesses-ptg-10015.html

The actual use of a PTG is quite straight forward - after first calibrating the device as per its instructions, simply place gently on cool paintwork and you will get a reading. The interpretation of the reading however is something which is much more difficult..

First of all, the actual number that you read is largely irrelevant - the exact number at least. What you should be looking for is the range in which that number falls to see if the paint is, in most basic terms: thin (<80um); normal (<150 - 200um); thick (>200um). This is of course a huge generalisation but it is these that we deal with with the PTG reading... So first thing to look at is the range in which your reading falls - is it alarmingly thin, pretty thin, normal, a bit thick, really thick? Very generally speaking:

• <80um: Thin, be careful, does this paint really need to be polished?
• 80<t<100um: Thin to normal - exercise some caution here but be aware this is normal for some cars (eg modern Jap)
• 100<t<150: normal, healthy
• 150<t<200: normal to thick, factory on some cars, query possible respary in which case exercise caution
• >200um: query respary, exercise caution

The second thing to look for is the consistency of the readings across the car. This can be assessed on a first go over the car performing a few cursory measurements (10 - 20 per panel), and looking for panels that exhibit either strangely high or low readings compared to the average on the car. This in many ways is more informative than the actual number as it will point to possible respray or it will point to a panel which on the car in question is thin... say you have a car which averages 130um, but on one panel you see 90um. This panel would be considered thin on the car and caution should be exercised. But on another car where the average across the car is 90um, and its a car with history of thinner paint, then this may be considered more normal... This is not an exact science and the readings you get should ver much be interpreted with care to maximise the information you can get from them.

The next thing to do with the PTG is take many readings on the panel - 100+! This may sound ridiculous, but on cars there can exist thin spots where there has been spot repairs and these can be the size of a postage stamp - taking only a few cursory measurements will result in you missing crucial thin spots that may cause problems should you opt for heavier compounding techniques... again look for consistency and regions that are notably lower or higher, mark out and tackle them with the caution they deserve.


Measuring Removal Rates

A hugely important consideration when machine polishing is how much paint you are removing from the car... Assessing this is not an easy task and exact readings are hard to achieve. However good indications can be had by correct use of a PTG.

One technique is to use a laser pen to pinpoint an area on the paint - measure thickness before, and then after and subtract to get the removal rates... This works, but I dont personally advocate this technique and much prefer the wider area average approach to assessing removal rates. This takes into accoun variations in thickness and gives a better interpretation of the average amount of paint removed which is far more instructive than an individual spot as removal is also not consistent across a typical machine polishing set.

To assess removal rates, find a landmark on the paint you are polishing - for example a stone chip (or use the laser pen* to create a spot on the paint as the landmark). Take between 30 and 50 paint readings over a 1" square region, and calculate the average of these readings (sum of the readings over the amount taken). After polishing, repeat this measurement of 30 to 50 readings over the same square inch and subtract the averages. This will give you a fair indication of the paint removed. If you can also calculate the standard deviation in your results you will get an indication of the consistency of the paint thickness.

Remember here that your gauge typically only measures to an accuracy of +/-3%, which equates to a couple of microns or more. Therefore if you're removal rate measured falls below a couple of microns you can essentially claim it to be inside reading error and you can make no sensible judgement other than to say removal was very low!

* Please exercise extreme caution in the use of a LASER. Most laser pens are either Class 1 or Class 2 laser devices. Class 1 is a "safe" laser with low power intensity and your blink reflex can (dont not rely on it!) save your sight. Class 2 (A & B) lasers are more dangerous - it is highly likely that should the beam hit your eye, your blink reflex will save your retina from permanent damage. But this should not be relied upon. Never stare into a laser beam and be very cautious of stray beams and reflections - lasers can permanently damage your retina, and harm your sight to the point where you will never be able to read again. Please... be very careful!



What Removal Is Safe?

You may think looking at the above diagram that you could get away with removing most of the clearcoat and not need to worry about it if you were tackling a deep scratch... but the truth of the matter is there is a lot more to consider than simply the raw thickness of the clearcoat and you should never assume that if 50um of CC exists that you can happily take away 49 of it!!!

Clearcoats provide UV protection and the amount of protection offered can decrease with the thinning of the clearcoat. This increased amount of UV through to the interface layer has been demonstrated in the literature to result in premature Clearcoat failure... in real world terms, thin clearcoat is more prone to peeling (clearcoat failure), and the thinner you make it, the more risk you are putting the paint finish in of premature failure. This is based on material published in literature.

More practically however, an everyday car is not going to stay perfect from when you finish polishing it and ideally you want to leave a good healthy amount of paint behind so that it can be polished again, and safely... 100% correction will look perfect until someone walks down the side of your car in Tesco's car park (dont think it wont happen!), then you'll need to repair the damage - of you've left the paintwork critically thin, good luck!!

In essence, the amount of paint removal that is safe will vary from car to car - but the less you remove, the safer it is. On both counts of maintaining the integrity of the clearcoat and providing scope for future correction should it be required.



So - Is 100% Correction Advisable?

100% correction - this seems to be what is aspired to by most who do machine polishing, but just what does this mean for your car's paintwork? Is it really safe to go for? Are there really tenable benefits?

First off, in order to restore the finish of a car to make it stand out from a crowd, attract the eye and restore colour and clarity, you do not need to remove every single little mark! 90% correction of all marks on the paintwork will result in a major turnaround on the car, and this would likely be achieved with the loss of 5um of paint or less... Getting another 5% correction (removal of some of the RDS) may cost you another 5um of paint, and the difference you actually see in practice becomes a lot less pronounced. Shooting towards 100% correction and you may well be looking at very significant paint removal to ensure every RDS, scratch, etching is removed... Now 100% correction is a great thing to claim you have, but in real world terms what has it bought you? There's no enhancement to clarity, gloss, depth - that came at the 90% level. All you achieve is the knowledge that some deeper scratches are also no longer there, and this may well have cost you a large amount of paint thus pulling the integrity of the clearcoat into question along with dramatically reducing the scope for future correction work to be carried out.

Being honest, achieving 100% correction is easy - keep polishing and you'll soon remove enough paint to correct a mark... but sensible correction is what you should be aiming for and should be the top most consderation when it comes to paint removal... how much is sensible to remove, and what correction is sensible to obtain given the car's future life. There's little point in removing 30um of CC on a car to get 100% correction (where 5um gave 95%) if the car is a hard working daily driver which is going to pick up marks again in the near future... not only is the CC integrity pulled into question, there is dramatically reduced scope to remove the marks that the car will inevitably pick up. Whenever carrying out paintwork correction, don't just think about getting the marks out - consider a lot more: what is safe; what is sensible; what achieves the results you are looking for without removing huge amounts of paint....

Obviously if one is considering a garage queen, then 100% correction is something more important, but again be very careful about what the work you are doing means for the paintwork in correction... As above, actually carrying out correction is comparitively easy... Carefully consider what you are doing and what this will mean for the paintwork you are working on is much harder and is to me far more the mark of a good detailer than raw correction alone.

So, check you're removal rates - does one hit with IP remove 5um of paint, get 95% correction and look superb? If so, does getting out Fast Cut Plus and battering a further 10um away for the sake of a few RDS make sense? It may do, on some cars, but does it on the car you are working on? I would urge great consideration of this, and pay very particular attention not just to how many marks you are getting out but rather the amount of paint you are removing and what this is going to mean for the life of the paintwork.

I add here that I am not suggesting we all stop going for the best correction we can - this is something every detailer, including myself, strives to achieve along with the best possible finish in terms of clarity and gloss... But what I am proposing is that far greater care is taken on the actual amounts of paint removed and that more than just outright correction is considered as to me there is much more to correcting a car's paintwork than this.

 

Wonderful thread Dave

What you have said is very true and this is why I do not believe in using only the Abrasive system for repairing paintwork
There is another system that is non abrasive and restores paintwork without removing any of it and is not oil based so filling of swirls etc are long term. at least one year.

many times, 95 to 99% correction is what I stick too but sometiems 100% can be done without removing hardly much more paint and in one step.
Rather than focusing on heavy levellers and compounds. softening of the paint to make it easier to correct and once finished polishing, it hardens the paint by 40%

it's an expensive system and takes some mastering but is definately a great asset and makes polishing/restoration of any car possible, no matter what the thickness

 

Nice reading on this article..

 

Great read, lots of valuable info

 

I agree, I just do daily drivers and I know the customers would rather have more clear coat and live with a few scratches.

 

Dude...If you ever get to Memphis, please look me up. :chips:


I will gladly Buy you a 1st Class Dinner just to be able to pick at your Brain a bit, Awesome write-up...:applause:

 

Dave..Wonderful thread!! DB'ers have heard me harp on this subject repeatedly. IMO you do a dis-service to customers when you grind down too much clear...and sadly they won't realize it until much later. Great job!!

 

so how much damage on average can a DA (flex 3401) do with an mild cutting pad (LC orange) and lets say M105 to be pretty agressive. Anyone know how many times a car can be polished with a DA and these methods and still be healthy? I know there is a ton of variables involved, but now i'm all worried and such about wanting to have achieved a high correction on my past 2 vehicles....guess i gotta suck it up and get PTG

 

Very helpfull info Dave Thanks :thumb:

 

Yep agree Dave, I'm glad you brought this issue up because customers have it far worse when their clear is chewed up on one or two details. 90% is the correct level of correction for even the weekend car.

My 350Z has been accumulating an imperfection here or there, but a good cleanse every 6 months keeps it looking in top shape. I'm in no hurry to polish.