Why Do Vehicle Paints Fade and What Can I Do to Repair It?

An Examination of What Paint Is, How It Degrades and What to Do When It Does

By William Rice

Vehicle paint systems are a combination of a pigmented color coat (referred to as the base coat) and a clear protective barrier coat, referred to simply as the clearcoat. Together, they combine to form a highly aesthetically pleasing, very durable, and relatively easy to maintain surface. As other articles in this magazine have pointed out, a good wash once a week, a good wax every month and periodic buffing to remove light scratches and blemishes, will keep the clearcoat shine and luster almost like new for years and years. Except, most vehicle owners don’t take that formula for keeping paint maintained to heart. People who wouldn’t think of not brushing their teeth at least twice daily, let vehicle paint go for months or even years.

The color coat of vehicle paint is not near as thick as it was when single stage paints were the norm. Then, you had 4 to 6 mils of paint to safely work with when buffing or sanding a paint imperfection. (One mil equals 1/1000 of an inch or 25 microns. By comparison a dime is over 50 mils thick.) Yet, that’s all the pigmented color that lies beneath the clearcoat of most cars today. (I’ve even heard from one insider that on some lines the color is only .8 mil.) The protective clearcoat is only 1.25 to 2 mils thick. Further, manufacturers can void the warranty if over .3 -.5 mil of clearcoat is removed by buffing or sanding. When the clearcoat is breached, it very quickly is GAME OVER for the pigmented paint below. So, what process explains why paint fades, oxidizes, discolors or chalks?

To understand the answer it is useful to explain what goes into a paint or coating since the same factors figure into the loss of color and gloss on any paint system.

All pigmented paints have four basic ingredient categories. Within each category there may be more than one ingredient. The four are: binder, solvent, pigment and additive.

The binder is the resin or blend of resins which give the paint most of its thermal, mechanical and weathering properties. The binder is the backbone of the paint, the foundation upon which every other component is built. Typically, binders are polymeric and are chosen by the formulator to give the optimum combination of cost, quality and performance profile. The solvent or solvents are referred to as carriers because they make the paint flow and “carry” the coating to the surface to be painted. When they are completely evaporated the paint is “cured.” Solvents, which can be oil based, water based or both, as when a water based paint needs a co-solvent that is miscible or compatible with water or dihydrogen monoxide, the chemical name for H2O. They can also have an effect on adhesion by what their degree of surface tension efficiency is.

The pigments are mostly nothing more than extremely finely ground mineral mined from the earth and dispersed into the paint. To be blunt, they’re really just colored dirt. Think of them that way and it’s easy to understand that when left exposed to the elements, without protection from

the binder, they become the proverbial “dust in the wind.” There are two types of pigments. The prime pigments give the paint its color and the extender pigments (or fillers) impact hiding, color retention, fungal and algae resistance, and durability. They both provide color opacity to an otherwise clear or translucent binder, weather resistance (elements such as titanium dioxide in either regular grade or micro-fine particles) and corrosion resistance to a paint. They are incredibly small. For instance, a particle of TiO2 is about 200 nanometers in size. You could fit 500 such particles in the width of a human hair. And, they are almost perfectly spherical when new. As paint ages, they lose that perfect sphere shape. What was once a marble becomes a golf ball and eventually a deflated furry tennis ball.

Additives are any modifiers that provide all kinds of special effects to paints, from rheology agents that make a coating flow out better, to ultraviolet absorbers and stabilizers that give it increased resistance to sun fading. They do some wonderful things; but, unfortunately, not for long. Way before a paint loses its integrity and starts to crack, peel or flake, additives are like Elvis, they’ve left the building. This is because the additive migrates to the surface and is dissipated over time rather than developing bond links to the polymer core for longer term stability. Obviously, the only difference between a clearcoat and base coat is the pigmentation. Now that we know the players, we can get to the heart of the answer.

If you examine a coating under a microscope you will find the polymer (binder) matrix can run the gamut from essentially straight or linear molecules (like a box of uncooked spaghetti) to highly complex molecules (like a coaxial cable wrapped inside another opposite directional coaxial cable) to anything in between. Much of their characteristics are determined by the molecular weight. Polyurethanes, like those used on cars and trucks, have a very high molecular weight. But, molecular weight doesn’t mean much if you don’t have complete reactivity of a material. In essence, reactivity is the linkages between the Part A liquid and the Part B liquid of a typical polyurethane coating. In fact, incomplete reactivity is the “dirty little secret” of the industry. They are always trying to get it better, but so far haven’t. Think of these polymer matrixes as being chains of molecules which get their strength from the bonds that hold them together (like links in a chain used to pull a heavy object). Whether simple or complex, they have one unalterable characteristic...they are chains that are only as strong as their weakest link. That’s why the reactivity is so important in urethanes. Reactivity is the chemical process that makes those linkages. Break the link bond at any point and you begin to weaken the chain that gives a coating its protective capabilities. Un-reacted bonds are “weak links” from the beginning. Don’t misunderstand, quality polyurethanes from any name manufacturer are good coatings, just not as good as some might like you to believe.

Thermal, photo-oxidative, mechanical, chemical, and biological degradation bring about changes in physical properties in polymers. They disturb or weaken the links that give the chain its integrity. There are other stress agents like airborne pollution, salt, sand, abrasion, solvents, biological contaminants, acidic or alkaline substances, chlorides, etc. that also add stress. But, as damaging as they are, they are only contributory elements leading to bond failure, or as some call it “polymer unzipping” because of the similarity to a zipper being unfastened. The big three are sun, moisture and heat, and more important than any one of them, is how they interact with each other.

Let’s start with sunlight induced damage known as photo-oxidation. Perhaps you’ve read about how “free radicals” are bad for you and how anti-oxidants of whatever kind (foods, vitamins, supplements) will control them and prevent damage to your body. Well, the same free radicals are part of this story as well.

First understand polymer backbones are made up of multiple elements. Key among these are hydrogen and oxygen. When the coating absorbs photons of UV radiation from sunlight, some of the energy “excites” the binder’s molecules to a higher energy level and causes bond cleavage, like a hot knife through the molecule’s heart. The result is free radicals. Free radicals are simply scavengers trying to steal an electron from a weaker molecule that has one. They react with oxygen in the air to create oxygen radicals, which then attack the polymer backbone. In so doing, hydrogen atoms break from the coating to form hydrogen radicals and more free radicals. This is where a vicious cycle begins. The free radicals combine with oxygen again to form...you guessed it, more oxygen radicals. The cycle repeats itself over and over in a chain reaction. More and more molecular links break...leading eventually to failure of the coating’s properties. Remember the demonstration of a nuclear chain reaction, where they had a room full of mousetraps all loaded with a ping-pong or golf ball. They threw a ball into the room and triggered a chain reaction. It took a slow motion camera to see clearly all the balls going off everywhere with increasing intensity. There you have, in its simplest form, the chain reaction that is photo-oxidation.

Moisture accelerates this degradation through a chemical process called hydrolysis. Once again, the process involves hydrogen when the splitting of a bond causes a reaction between the hydrogen from the coating and the hydroxide from the water (H2O).
Finally, the infrared heat energy from the sun comes into play, expanding and contracting the coating during a 24-hour cycle, causing accelerating stress fatigue degradation. High temperatures also accelerate the process of link breakage outlined in photo-oxidation.

All these stress factors lead to one additional form of degradation, the porosity of the coating film. It’s like having an umbrella attacked with hairpins. The holes created are small and relatively insignificant until they become so numerous that the integrity of the umbrella (coating) is compromised. Once moisture, with all the contaminants it can bring along, and oxygen (that’s why they call it oxidation) gain access to the coating’s weak underbelly, the coating’s protective barrier (whether clearcoated or not), is attacked from inside and under to devastating effect. The paint’s pallbearers with a trumpeter playing taps won’t be far behind.

What to do?

For most cars and trucks, compounds and waxes are fine as a temporary “quick fix” that doesn’t cost much. And, we all know, cost is a big factor. But, look at the problem from a different perspective, one more long term. We know the base coat has color but no color intensity or gloss on its own. It relies entirely on the clearcoat to give it a deep luster. Every non-clearcoated paint or fiberglass gelcoat has the same profile when it fades, dull color with diminished or no shine. So, whether clearcoated or not, the real solution is to put on a new layer of clearcoat.

I know you’re thinking, it’s not practical. For cars and trucks…maybe it’s not. But, that doesn’t change the fact that what you want to do for a badly faded surface is restore the original sphericality to the pigment(s) and replace their moisture content. That restores like new color. The clearcoat also gives the paint gloss or shine. If you want to test this notion, take something badly faded and apply an ounce of Mazola corn oil to it. It looks pretty good, doesn’t it? Of course, it won’t last more than a day, which just emphasizes the importance of choosing a clearcoat for long-term durability.

Think about offering your customers, and people who aren’t now your customers, something new. Be their clearcoat detailer. Check supplier training, on-going support, and reputation. Use your wash bay at night when it is not making you any money. Maybe start a mobile business. Bottom line, you can make a great deal of money doing something really needed, restoring the original color and gloss to almost every dull, faded, oxidized surface.

To illustrate, I’ve included a before and after photo I’m sure you can relate to. It shows the hood of a 1992 Jeep that recently came into our shop in Atlanta. The clearcoating was peeling badly. We washed it thoroughly, and then wet sanded the surface until all the loose clearcoating was removed. When we sprayed it with our clearcoating, all the paint was restored. Not a perfect job, I’ll admit; but, an adequate one with a very pleased customer.

Hopefully, I’ve given you a better understanding of the paint weathering process.