Understanding, wet sanding and buffing scratch resistant clear coat

 

By Kevin Farrell of Kleen Car Auto Appearance

 

There has been much talk recently regarding the newer scratch resistant clear coats. Some are very new and some are a few years old already. However they are here to stay and most car manufacturers will be producing more and more cars with this newer clear coat technology. 

Although these clear coats are not bulletproof, the scratch resistant clears do hold up well to light scratches, stay cleaner as they do not let as much dirt penetrate the surface, and some of them actually “heal” or remove light scratches when left out in direct sunlight. It may seem that these clear coats will put detailers out of business, but that’s not the case. They still need care and they still will need to be buffed out when imperfections get deeper into the surface.  

A little bit of chemistry

I always hated chemistry. It was confusing and really kind of boring. So in an effort to educate but not have you turn the page out of sheer boredom, I will try to keep this simple. When a clear coat is being applied or sprayed, it needs to react with something to become the hard, shiny finish that you see on most new cars. Some use what’s called a 2K or 2 component system that starts to react and cross link when the 2 components are mixed together. If you combine this with heat or baking in an oven, the clear coat cross links faster and to a higher degree, creating a hard and tough surface.

The problem many car manufacturers had years ago when they were using a different version of a harder clear coat called a Melamine clear, was that it was very brittle and was very susceptible to stone chips. It also did not weather very well. If the vehicle was in a climate with drastic temperature changes, it would have problems with its elasticity and it could eventually crack. It also was not very etch resistant, meaning that acid rain would penetrate the surface and etch the clear, as would bird droppings, tree sap, etc. So this version of clear coat gave way to softer versions that had more elasticity, more gloss, more etch resistance, retained a better gloss over a longer period of time and held up better to weathering. The down side with the softer clear coats was scratching and marring. They just do not hold up very well to any kind of scratches. 

But now with the newer technology, we are back to harder, more scratch resistant clear coats that meet the car manufacturer’s requirements on everything previously mentioned.  

Ok, here’s the technology, hopefully in simple to understand language, as I needed both David the owner of Optimum, the people I work with at BMWNA and other paint chemists to “dumb this down for me”.  

I am sure many of us have heard that most clear coats are of a poly urethane technology. It’s basically a form of plastic, but with more elastic capabilities. The way they get there is to combine an isocyanate (difficult to understand and no need to explain) with an acrylic polyol (also difficult to understand and no need to explain). These 2 components make up the 2K in the 2 component clear. However, the new technology of scratch resistance adds something. It adds a hydrogen bond. The hydrogen bond allows some thermo plastic flow making the clear more flexible to relive stress, not chip or crack, and also to actually “self heal” and kind of repair itself. What helps in the hardness of this clear coat is the level of cross linking of the 2 components as well as baking it to further increase the hardness. Many of the scratch resistant clears are baked at about 280-290 degrees for about 20 minutes to achieve full curing and cross linking. A factory baked scratch resistant clear coat can sometimes be very hard and truly a challenge to buff and/or wet sand. 

These clear coats have a tighter cross link density. Think of it like tying your shoes. If your shoes are loosely tied, there will be more gaps between the laces and a greater distance between both halves of your shoe and the shoe will be very loose. If you pull your laces really tight, there will be very little distance between both halves and the shoe will be much tighter. This also creates more surface tension. The surface becomes harder as a structure and also much more difficult to penetrate. This is also another reason why these clear coats are more difficult to scratch, etch and mar. It’s also a reason why they seem to stay cleaner. Dirt cannot penetrate as easily and that keeps the surface brighter and cleaner.  

Some versions of these clear coats with these characteristics and properties are called Re-Flow scratch resistant clears. I didn’t quite understand this terminology as “re-flow” would mean that heat created will soften the clear. You may ask, as did I, how can a clear coat that is supposed to “re-flow” and be more elastic also be hard and scratch resistant? I assumed a technology such as this would make it softer, much like the clear coats of the 90’s and early 2000’s.  

The theory of scratch resistance of a reflow clear

Since I am sure we are all rather chemistry challenged, here is how it works in hopefully easy to understand terms. When the 2 components mix, they start to cross link and that adds structure and strength. The urethane structure gives the coating good chemical resistance and weather resistance, as well as hardness, toughness and elasticity. A unique aspect of polyurethane chemistry is that the hydrogen bonding acts as an additional crosslink, but also allows for the thermoplastic flow to relieve stress, not chip and enable self-healing of minor (yes minor) scratches.

The basic structure of a polyurethane clear coat features a soft segment, based on the polyol, which gives it flexibility and elasticity. There is also a hard segment that has high urethane density, which gives the coating hardness and toughness. 

I was able to understand this because of my hockey experience. Who knew? I play both ice and roller hockey. Obviously in roller hockey the skates have wheels. They are also of a urethane, more like rubber (this will be important to remember when we get to wet sanding this stuff). They grade the wheels based on what surface you are skating on. Some wheels need to be very hard and some need to be softer for that particular surface. They grade the hardness or density of these wheels in a term called durometer which I have needed to understand to get the correct set of wheels on my skates. The higher the duromerter or density, the harder the wheels will be. Some of the harder wheels actually are hard as a rock and they take much longer to wear. They are more brittle and have much less flex. The softer wheels have a lower durometer. They wear very quickly but have great grip. 

Almost sounds like some of the different versions of clear coat doesn’t it? Clear coats can be made with harder durometers or softer durometers and that will also determine the scratch resistance. All of this combined will give the clear coat a level of scratch resistance, yet at the same time will make it self healing and have a “reflow” effect. To fully understand the theory of scratch resistance, we must know what types of scratches this clear can self repair or heal. Only a lighter scratch or a mar in the clear which would be a typical wash scratch or small abrasion that does not result in whitening, can have the ability to “heal” itself. When the clear coat gets a deeper scratch it will show up as white. This is the chemistry in ANY clear coat. When the scratch is white, the clear coat has been “fractured”. This type of scratch will not be self healing. A light mar or scratch that only “deforms” the clear coat can technically self heal and repair itself (sometimes only to a certain degree) when heat is induced and it reaches a certain temperature. It will reflow and “theoretically” flex and contract and significantly improve the scratch or make it disappear.  

A key scratch or a deep white scratch is an example of the clear coat being compromised and “fractured”, and thus not able to self heal or reflow. More conventional buffing methods or possible wet sanding would be needed to improve or eliminate this type of scratch. But there is more. There is still another totally different type and chemistry of a scratch resistant clear coat.  

Cerami-Clear or nano particle clear

This is another clear coat that PPG developed for Mercedes a few years ago. This type of clear coat has slightly different technology and properties compared to the reflow and high cross linking scratch resistant clear coats. In this system, PPG uses nano (very small) particles of ceramic, which is obviously a very hard material, to gain its scratch resistance. Here the nano particles of ceramic migrate to the very top portion of the clear coat, usually within the top 3-5 microns. It’s not a very dense layer but is effective at giving the same type of scratch resistance to similar marrings as the other versions of scratch resistant. In other versions of this clear coat made by other paint manufacturers, a particle called fumed silica is commonly used and incorporated into the clear the same way.  

This clear coat is very hard at the top where the hard nano particles migrate. However once this layer is broken, the clear coat becomes much softer and has very little scratch resistance below this layer. There is no concrete consensus on which version of clear coat is better, or for that matter, what company makes the best version of a scratch resistant clear coat.  

Whose clear coat is the hardest?

I wish I knew. There are some hard clears out there. There are both versions on the market in both factory finishes and refinish versions. Some are harder than others. The more confusing issue is that even in a company that is using scratch resistant clear coat across the board, some still will be harder than others. Sometimes it’s tough to tell what brand is being used within a manufacturer. All car manufacturers have many plants where vehicles are built. Some of those plants use different brands of clear coat. BMW uses at least 3 different brands of scratch resistant clear coat on cars made in Germany. All have different characteristics and varying levels of hardness. The same is true at GM. In all their plants, there are many different brands of clear coat being used. Many have very distinctive characteristics regarding hardness.  

You may ask, why don’t they use just one brand? Or why are they all so different? This just makes our jobs as detailers that much harder to work on so many different versions of clear coat. As long as a paint manufacturer meets or exceeds the factory specifications on what they want in a paint system, regarding scratch resistance, elasticity, gloss, etch resistance, etc., the various scratch resistant clears are approved for use. Some of these clear coats may just meet the hardness requirement and scratch resistance and some may far exceed the requirement. This leads to different paint systems that will not buff out the same way even though they are still all termed as scratch resistant clear coats. While a scratch resistant clear coat will be harder to mar and have deeper scratches, it will still need correction at some point. This is where it becomes interesting. 

In our next issue we will discuss sanding and buffing of these scratch resistant clear coats and why different methods and products may have to be used.