Abrasives in Car Care: Kaolins for polishing paintwork

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Car paintwork consists of three layers: Primer, base coat and clear coat. Beneath these is the metal body panel. Abrasives can be used to provide physical surface preparation and cleaning, aiding in the levelling of the clear coat layer. This allows for a cleaner and newer-looking finish to the paintwork as the smoothing of the clear coat layer creates a high gloss finish, free of imperfections. Most polishes contain kaolin clays, diatomaceous earth and/or alumina (Al₂O₃).

What happens to paintwork as it ages?

With exposure to UV light, fluctuating temperatures, oxygen, stone chips and general dirt, car paintwork begins to break down both physically and chemically (via oxidation) over time. As this happens, the paint becomes duller, losing radiance and taking on a chalky/faded appearance (figure 1). This makes a car look worn and aged with a variety of surface defects occurring.

Figure 1: A comparison of heavily oxidised paintwork before and after the paint has been corrected with an abrasive.

How do abrasives work in Car care?

The ability of an abrasive to clean and smooth out surface imperfections depends on the particle size, its morphology and its Mohs hardness. The Mohs scale of mineral hardness is a qualitative scale, from 1 to 10, characterising the scratch resistance of minerals through the ability of a harder material to scratch a softer material. The larger, less uniform and harder the abrasive particle, the greater the ability to remove the oxidized paint layer. Milder abrasives are composed of more uniform, smaller particles, which polish more and clean less, giving greater gloss. These are better suited to surfaces with little or no oxidation.

Figure 2: Cars are polished using polishes with varying abrasive ability in order to get the best results for the type of paintwork imperfection.

What distinguishes different types of polishing compounds?

Cutting and finer polishing applications differ from one another in the intensity of the products used for the desired result. A coarse cutting compound, for example, will be more abrasive and remove more of the surface material, faster. A finer polishing compound or polish will have a much less aggressive level of cut, and be designed to smooth out the surface on a finer scale, inflicting tiny scratches that the human eye can no longer see. There are a number of similarities to sanding down wood. Coarse sandpaper will take away a large amount of material quickly, but leave a noticeably unrefined finish (visible to the human eye). Following up with varying degrees of finer sandpaper smooths the surface to a level that's pleasing to the touch and pleasing to the eye. It's exactly the same idea when polishing paintwork, just on a smaller scale. Abrasion properties can be given by the Einlehner Abrasion Value measured using Breunig’s method. This measures the weight loss of a bronze wire after a 15% abrasive slurry has passed the mesh under prescribed conditions e.g. Satintone 5 HB was measured around 4-5 mg which is low (gentle abrasive).

Examples of abrasives for Car care

Table 1: A comparison of the different types of abrasives with their Mohs hardness and their median particle size.

KaMin Satintone 5HB Calcined Kaolin

A small particle size and a middling Mohs hardness value makes Satintone from KaMin excellent for gentle abrasion. A gentle abrasion suits car polishes where the removal of light scratches, scuffs and marks alongside the restoration of gloss is required. Satintone 5HB can also be combined with more intensive abrasives to help smooth out their effects.

Figure 3: Scratches on the paintwork of cars can be nearly impossible to remove without the proper equipment. Polishes that are designed with this purpose in mind will have an abrasive element in.

KaMin Mattex Pro Dehydroxylated Calcined Kaolin

Increased hardness and a larger particle size make this a more abrasive alternative to standard calcined kaolin. This would be more suited to a medium-cut, rubbing compound-type formulation.

KaMin ASP 072 Hydrous Kaolin

ASP 072 offers the only hydrous kaolin option on this list. It has a low Mohs hardness and very small particle size so is ideal for a fine polish to smooth out the appearance of the paintwork.

HPF Quarzwerke Tremin 283 600 Wollastonite

As the only non-Kaolin abrasive on this list, Tremin 283 600 from HPF Quazrwerke is larger, less uniform and blocky with a rod-like particle shape. This makes this a more abrasive alternative to the kaolin options, despite having similar Mohs Hardness. It would be ideal for a medium-cut polish.

Figure 4: The morphology of Tremin 283 600 showing the rod-like particle shape.

Summary

Abrasives are widely used across the car care industry to restore the original shine and finish to paintwork, quickly elevating the appearance of a vehicle. When selecting an abrasive material, it is important to understand its unique properties. There is no ‘’one size fits all’’ solution. An understanding of the desired level of cut you’re looking to achieve and the factors influencing this property is essential to get the right product or blend of products for the job.

If you would like to discuss your specific needs and requirements, please don’t hesitate to get in touch to discuss your project or request samples.