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Blame it on the acid rain, Part I

October 11, 2010
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It’s a tough world out there for a vehicle’s paint finish. It has to be protected from the salt, rust, flying gravel, dirt and the biggest culprit, acid rain. No vehicle is totally resistant to acid rain and with each cycle of rain, bright sun, and high temperatures, acid eats away a little more paint film getting through to the metal.

Hopefully, the problem is caught before the acid eats completely through the paint. On today’s basecoat/clear coat finishes, it rarely gets through the clear coat. However, the clear coat will develop unsightly shallow craters known as etching.

This article is the first in a two-part series about how to prevent acid rain from etching paint, as well as how to repair the clear coat damage caused by acid rain. In most cases, damage can usually be repaired without the need to repaint the vehicle.

Please note that I do not endorse any particular paint etching repair process as there are many products and systems that are available from different manufacturers, all of which are very similar. The important thing is to precisely follow the instructions for each product used and to not assume that one product is similar to another and can be used in the same way. Odds are that they cannot.

You might be thinking at this point, “why not use a buffer, compound and cutting pad to repair etched paint?” The problem is that these methods and materials tend to round off the edges of etch marks without really removing them. As a result, the imperfections stand out instead of blending in. To solve the problem, the etched areas must be leveled, or evened out with the surrounding areas to blend them all together. The repairs should result in a smooth, high gloss, mirror-like finish. This can only be achieved using a more intensive process than buffing with a compound.

Characteristics of base-coat/clear coat finishes
These paint systems were designed because they are tough and durable, and give the effect of rich, deep color that always looks freshly waxed.

A base-coat/clear coat finish is made up of three distinctly different layers. First, the metal body panel is initially coated with a primer surfacer which seals the panel surface and provides a good base on which the subsequent coatings will stick. Next, a urethane enamel base-coat is applied. This layer contains little solvent and is made up mostly of solids (thus, it is referred to as a “high solids” coating).

The base-coat is the only layer that provides color. It can also be known as the “color coat.” Because the base-coat is a high solids coating, it dries to a dull finish. The final layer is a urethane enamel clear coat. The clear coat provides the high gloss “wet look” to the car’s finish. It also protects the base-coat from the sun’s ultraviolet (UV) rays.

The thickness of the clear coat is very important because if too much of it is removed through buffing or sanding, the ultraviolet rays will cause the paint in that area to crack, peel or fade. Typically, the clear coat applied at the factory is at least 1.5 mils thick (a mil is 1/1000th of an inch). Actually, for long lasting protection only 1 mil of clear coat is required.

The paint repair procedures discussed in these articles should allow you to successfully repair acid rain damage without repainting either the clear coat or color coat. But do not remove more than 0.5 mil of clear coat. If you do this, it may be necessary to respray the clear coat to restore the needed UV protection. And, if the base-coat is disturbed, a complete repaint might be necessary. Without question, a detail technician must know what they are doing, and of course, when to stop doing it.

Understanding the chemical etching process
The emissions from industrial plants and automobiles release sulfur dioxide and oxides of nitrogen into the atmosphere. As they spread through the air, they are picked up by moisture and fall back to the ground in the form of rain drops, mist, fog or dew. By the time these drops land, they have become diluted solutions of water and sulfuric or nitric acid, commonly called acid rain. Because they are diluted they do not immediately etch the vehicle’s paint. However, other factors boost the acidity of the drops to the point where they will etch the paint everywhere an acid rain droplet lands.

For example, once exposed to bright sunlight, the water droplet begins to evaporate. As the water vaporizes, the acid content in the remaining solution becomes much higher. If the day is hot and the sunlight strong, the rising temperature of the body panel beneath the paint can dramatically increase the effect of the acid. The darker the paint, the more sunlight it absorbs, and the hotter the body panel will be. Depending upon the geographic area, the strength of the sun, and temperature, acid rain paint etching may range from a few minor spots that can barely be seen, to numerous shallow craters you can actually feel.

In most cases, acid rain etching occurs on the horizontal panels of the vehicle, like the roof, hood, trunk lid and tops of the fenders. Why these surfaces? Because water beads up and the droplets remain on these areas. This allows the sun and heat time to create a paint-eating droplet, and it gives the acid time to do its “dirty work.”

Acid rain is a worldwide problem, but it is not the same in all areas. Generally, the greatest acid rain damage occurs in the northern parts of the United States during the months of June, July and August. In the southern parts of the country you will have to be on guard against acid rain etching for more than three months of the year.

Acid rain is not the only thing that will etch a vehicle’s paint finish. Dry pollutant particles can settle on the vehicle’s finish and cause damage. These dry particles react with the water vapor (humidity) in the air or with rain, mist, fog or dew and will etch the clear coat in the same way as rain.

So how do you rectify the various kinds of chemical etching? The type caused by acid rain looks like ordinary air dried water spots, except that the spots will not rub off. On the other hand, the type caused by dry particles may form small pits, or pin holes in the paint surface. Advanced damage from dry particles look like small, shallow, half moon shape craters. Without preventative measures, either type of chemical etch damage can become severe enough to expose the basecoat of the paint. Note that the above damage is best observed under fluorescent lights.

Etching can also occur from bird droppings, insect residue, and eggs on a vehicle that might have been “egged.” Customers need to get these materials off the paint as soon as they are discovered to prevent etching.

Preventive measures
Many of the auto manufacturers have attempted to reduce or eliminate chemical etching by covering the paint finish with plastic film during shipment or applying a transit coating to protect the vehicle for the first 30 days after painting when the finish is not completely cured. If the finish does get exposed, even to non-acidic rain drops, some of the resins, activators or solvents in the clear coat may combine with the droplets and form weak acid solutions. Add acid rain to those solutions and chemical etching becomes even more likely in this first 30 days.

The plastic film is a 1-2 mil thick adhesive backed product that adheres to the finish until removed.

The transit coating mentioned, forms a milky, wax-like barrier on the paint protecting it from chemical pollutants. Typically, it is left on the vehicle 30 days, but not longer than four months after delivery to the dealer. After four months the coating can interact with the clear coat, and may cause an irreparable discoloration that often cannot be removed.

Part 2 of this series will discuss the repair process.

R.L. “Bud” Abraham is president of Detail Plus Car Appearance Systems, Portland, OR, and a nearly 40-year member of the car-care industry. He is also a member of the International Carwash Association and Western Carwash Association Board of Directors. Abraham can be contacted at