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jahk86 08-12-2016 08:56 PM

Machine Polishing/Compounding PTG
Say I want to machine polish a bonnet using a DAS PRO PLUS with 15mm throw, what is the general rule of thumb on returning to this area again. How many times can you give an area the paint correction treatment before you have removed too much clear coat.

I know this is a broad question which leads me onto my next question.

I've read the only way to truly know is to use a PTG, which one is good for someone who does not want to break the bank but needs something in their tool kit as a beginner, and also in terms of the readings when is it too low to even think about reaching for the compound?

Looking for a general rule of thumb for when you don't have a PTG, and a recommendation for a good entry level PTG and some info on what the readings mean?

gaswizards 08-12-2016 09:48 PM

Hope this helps ( this is part of the da detailing guide on here):

Most machine polishes use mechanical abrasives to remove the paint defects. This means that as you are polishing, you are eroding away a certain amount of paint, the exact amount depending on how harsh an abrasive you are using.

If possible, before starting to use the machine polisher, it is a good idea to check the thickness of the paintwork on the car. This can be done by using a Paint Thickness Gauge (PTG):

Most PTGs will measure to total thickness of paint on a panel. Paint generally consists of either two or three distinct layers: Base Coat; Colour Coat; Clear Coat (on most cars). It is the thickness of the top layer of paint which is of interest – going through this layer (strike through) will result in a respray being needed! So it is necessary to interpret the readings you see on the gauge and a certain amount of guess work will be required. Very generally speaking:

This level of thickness can be expected on older cars that have been hand painted – the Ferrari F355 for example. It is also indicative of the possibility of the car having been painted at some point in its life. This is something important to note as non-OEM paint can respond very differently to factory paint in terms of hardness and polish behaviour – so if any regions appear quite thick, make a mental note of this for when you come to machine polishing them.

An example of just how thick paint can be is seen in the picture below where a reading of over 1000um was taken – that is paint that is 1mm thick!

This is normal paint thickness. Reading in this range point to standard paint (generally speaking) and shows a healthy thickness that should present no problems when it comes to machine polishing.

For most cars with clear coats, readings in this range point to quite thin paint. Many newer cars give thinner paint readings, but reading in this region should raise caution. Think twice about using more aggressive polishes on thinner paints as significant amounts can be removed, resulting in strike through and the need for a respray. Paint of this level can still be machine polished – but greater care should be exercised in polish and pad choice.

Now we are in the realm of very thin paint, especially if a clear coat is present. Care should be taken when choosing a polish for these thinner paints. A big consideration should be a filler heavy polish which can achieve correction by filling the marring rather than removing any further paint. Thin paint can be a result of aggressive machine polishing in the car’s past life.

As said at the start of this section, it is the thickness of the top layer of paint which is of most interest for assessing the suitability of using machine polishes. But with a reading of only the total thickness, how can you know the thickness of this top layer? The answer is that, unless you buy a PTG that can measure the individual layers (expensive!), you cannot know to high accuracy – but you can give an educated guess.
50-25-25: Very generally speaking, on most cars the clear coat makes up 50% of the total thickness. This is a general approximation, and only a first very rough estimate.
Inside of door: Measure in here. This will give you the thickness with much less clear coat sprayed and you can assume this to be the thickness with no clear coat to a pretty good approximation. The difference between readings inside the door and on the exterior paintwork gives the thickness of the clearcoat.

jahk86 08-12-2016 11:02 PM

Do you have a recommended PTG, there are many out there?

lowejackson 08-12-2016 11:12 PM

Polishing without a PTG is guessing but this needs to be put into context. I would guess the vast majority of people do not have a PTG and the forum is not full of pictures of damaged paint. If you know the history of your car, the odds decrease in hitting a problem.

Of course, none of this matters if you hit primer whilst you are polishing......

DrEskimo 08-12-2016 11:14 PM

The idea of all of this detailing melark is that you carry out a safe wash routine that results in little to no damage. Coupled with a decent coating for protection, there should be no reason to have to carry out a paint correction on a semi-regular basis.

I would borrow one if you can, make sure the paint is all good if the car has an unknown history (great for telling you if any panels have been repainted), carry out the correction and then hopefully never have to correct again.

jahk86 10-12-2016 10:05 PM

I think what I mean is, I don't have a full history on my car of each and every time its been machine polished. I have no idea other than as part of the approved BMW purchase they machine compounded it as part of the prep.

I have since hand polished a few areas.

Now I have a DAS6 PRO PLUS, I would like to do a full panel to really remove some of the imperfections I couldn't achieve by hand.

I don't want to tape off the bonnet, start a quarter and immediately burn through the clear coat, because unbeknown to me it has been done by the previous owner....?

So, do I take a risk and do a full paint correction then retain this through proper cleaning techniques, OR do I buy a PTG and check before I start machine polishing?

Kickasskev 10-12-2016 10:44 PM

I use the painted area on a door shut as a guide, for example

If the door shut has 60 microns I would know that going beyond the 60 on a panel would be very risky, if the panel was 100 micron I know that I would roughly have 40 microns to play with, the reason being the door shut usually has the thinnest amount of clear coat, this is the guide I use and it hasn't failed me yet, but always check in case the panel you are polishing has been previously painted, if so this guide would be no good and you should really take caution.

Kickasskev 10-12-2016 10:51 PM

Chance of burning through with a DA are very slim to zero, I quite often have to revert back to my machine polisher because I cant remove all scratches with the DA as they are so gentle

suspal 10-12-2016 11:49 PM

I've seen people strike through with a DA,it depends on the state of the paint and common sense,although a DA is a safer bet as the way it spins and relative slower speeds than a rotary,lets say pad,polish/compound and technique plus a ptg are king.

diggy87 11-12-2016 12:15 PM

I'm toying with the idea of a PTG myself and keep looking at this one -

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