Paint, just like any solid surface, has a certain hardness. To put it really simple, this tells you the force needed to put a scratch in the surface. Very soft paint gets marked easily, but is also easier to polish. A hard surface might not get scratched as easily, but will also be more difficult to polish.
Why is there a difference in hardness
The hardness of paint can come from a few different causes. The way it was dried, the thickness of the layer of paint, the type of paint used, the chemicals that where used in the paint etc. It is very difficult to tell why a certain car has harder paint then another car. Even more so, just like with the paint thickness, the hardness can very between one panel or the other.
Why does the hardness matter
When you going to polish a surface, you will basically remove a very thin layer of the surface and at the same time try to leave behind a surface that is as smooth as possible. When a surface is very hard (for example: glass) it takes much more time and effort to remove this layer. If the paint is very soft, it will be very easy. However, that would also mean that very hard paint needs a rougher polish and a polishing pad with more cut. This will result in a combination that polishes the surface with much more force and effect then a soft polish with a low cutting pad.
The hardness of the paint therefore dictates the polish, polish pad and to an extent, even the machine you should use. Knowing how hard the paint is can also help you to know when to stop polishing and when to start wetsanding.
Paint hardness with different brands
Some people might be convinced that German cars all have very hard paint, and although this may be true for many German cars, it’s not true for all of them. There are several reports from detailers encountering German cars with soft paint. The same can be said about Asian cars. The general conception is that these all have soft paint. But there are also reports of Asian cars with very hard paint.
It would be very difficult to make an accurate list of how hard the paint is very any given car brand and/or model. If the painter decided to mix the paint slightly differently, you will have a different hardness than the cars before or the cars after. Brands such as Lamborghini have changed ownership which can also cause the type of paint to be changed.
For example: Rover has made many models that where painted in Britain by their own paintshop, however the Rover 75 was painted with BMW paint, which is different. Most of the Rover cars will have normal or relatively soft paint, but the Rover 75 has very hard paint.
Determining the hardness
There are three main types of hardness measurements: scratch, indentation, and rebound. Within each of these classes of measurement there are individual measurement scales. For practical reasons conversion tables are used to convert between one scale and another.
Scratch hardness is the measure of how resistant a sample is to fracture or permanent plastic deformation due to friction from a sharp object. The principle is that an object made of a harder material will scratch an object made of a softer material. When testing coatings, scratch hardness refers to the force necessary to cut through the film to the substrate. The most common test is Mohs scale, which is used in mineralogy. One tool to make this measurement is the sclerometer.
Another tool used to make these tests is the pocket hardness tester. This tool consists of a scale arm with graduated markings attached to a four-wheeled carriage. A scratch tool with a sharp rim is mounted at a predetermined angle to the testing surface. In order to use it a weight of known mass is added to the scale arm at one of the graduated markings, the tool is then drawn across the test surface. The use of the weight and markings allows a known pressure to be applied without the need for complicated machinery.
Indentation hardness measures the resistance of a sample to material deformation due to a constant compression load from a sharp object; they are primarily used in engineering and metallurgy fields. The tests work on the basic premise of measuring the critical dimensions of an indentation left by a specifically dimensioned and loaded indenter.
Common indentation hardness scales are Rockwell, Vickers, Shore, and Brinell.
Rebound hardness, also known as dynamic hardness, measures the height of the “bounce” of a diamond-tipped hammer dropped from a fixed height onto a material. This type of hardness is related to elasticity. The device used to take this measurement is known as a scleroscope.
Two scales that measures rebound hardness are the Leeb rebound hardness test and Bennett hardness scale.
Ceramic coatings and their hardness
Many ceramic coatings advertise with the hardness of their coating. Some even claim to have a hardness of 8 or higher (Diamond is 10). This means that the ceramic coating might be harder then the paint it is applied on. This offers very good protection, but it also means that removing the coating or correcting imperfections is much more difficult. The biggest missing link in their claims is the absence of scientific test results, carried out by an unbiased and independent party.