Does a paint thickness gauge measure just the clearcoat thickness though, as opposed to the primer/basecoat/clearcoat combo, or is an assumption made as to the relative thickness of these? Not trying to be controversial, just interested as to how these things work and how detailed and reliable the measurement is.
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There is / was a cracking post on detailing world outlining all of this, it's still there but the pics, which tell a thousand words are all photobucket and hence GONE.....See below...
HOW MUCH PAINT? 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. Rotary polishers have greater power than their Dual Action counterparts and for this reason greater care should be taken with regards to paint thicknesses – especially with more aggressive cutting compounds!
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:
200 MICRONS + 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.
BETWEEN 100 AND 200 MICRONS 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.
BETWEEN 80 AND 100 MICRONS 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.
LESS THAN 80 MICRONS 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.
LAYER THICKNESSES? 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.
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