A Painting Primer
By Mark Hall - Rio Grande Redline
reprinted in Tarheel Chapter, *Footnotes
Having recently purchased a 1980 633csi that is sorely in need of paint, I thought I would share with you some of my research into what it takes to get a good paint job. While the factory lays down an outstanding cover for the bare metal on your car, exposure to tar, salts, road grime and the sun's harsh ultraviolet rays will take its toll over time.
First, the paint itself...There are three basic paint types, lacquer, enamel, and urethane, each with their own benefits and drawbacks. Laquers are quick drying and easy to repair, but must be wet sanded and buffed, and generally require more than two coats to properly cover. These multiple coats also make it vulnerable to cracking. Enamels (and acrylic enamels) require only one to two coats to cover, need no rubbing or buffing and dry to a hard, durable finish. They resist ultraviolet oxidation and environmental hazards better because they dry much harder.
Enamels take a long time to cure, however, and require almost perfect application conditions because nibs of dust and dirt cannot be rubbed out--such a procedure would break the outer paint skin and mar the surface.
Acrylic enamel can be sprayed over most lacquer or urethane paint with no problem, but in most cases, lacquer cannot be sprayed over enamel (what I believe happened to my 633). Urethanes are one of the newest types of paint for automobiles. For years they have been used in the airline industry for their superb durability. DuPont's Imron brand is a urethane paint. Since it was one of the first of these types of paint to reach the auto market, many people mistake the brand Imron to mean a urethane paint type. Many other manjfacturers like PPG, BASF, Sikkens, and others produce the same type of paint, but label them with their own brand names.
BMW's like my 633 used a two step painting system called base coat-clear coat. The base coat is the color and the clear coat is just that - a clear coat of point that helps to give the base coat a deep rich gloss. When properly applied, multiple clear coats can really make a paint job look better that most any other method. After application over the base coat, the clear paint is wet sanded and rubbed out to bring out the shine. Sanding and rubbing should never take so much of the clear coat off that you have to touch the base color coat of paint.
Regardless of the type of paint you choose, make sure you (or your selected paint professional) use products from the same manufacturer throughout the painting process. Not all clear paint products are compatible with all color coats. Although opinions differ as to which brand of paint is the best, there is no question that when using a distinct paint brand, you should use the same brand for base coat, clear coat, and any related thinners, primers, reducers, etc. Metallic paints, like the Chestnut Red on my 633, are nothing more than tiny flakes of metal added to the clear coat. As light reflects off these particles on top of the base coat, it gives the paint a "sparkling" quality. Because of their reflective properties, metallic finishes may redirect sunlight onto the paint surface to which they are applied, causing oxidation problems. Highly metallic finishes like silver and light metallic blue seem to have this problem more than other colors.
Once you have selected the right type of paint and color for your needs, the key to any good paint job is the preparation of the surface to be painted. You need to ensure that it is completely free of all imperfections, since even the slightest hint of a door ding or dimple will appear to be magnified 10 times when the car has been freshly painted. Any quality painting professional will not even consider painting a car whose body has not been smoothed out.
I plan to get the 633 painted at the end of this month. I will be removing all the body hardware (chrome, door handle, bumpers) myself to save a few bucks. I am paying a professional to completely strip the old paint down to the metal (a necessary evil due to previous owners complete lack of care). I'll report back in the future with some lessons learned and let you know how it turns out!