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Paint Materials Guide - Color Chemistry

Understanding Paint Top-Coat Materials

Steve Dulcich May 25, 2010
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When it comes to the world of automotive refinishing, it seems as though the varied specialized products are nearly endless. When considering the types of primers below and the color and possible clear coats that may follow, there certainly are a wide range of product types, and it pays to become familiar with them. Each manufacturer of paint products offers their own specific line of primers and paints, providing a "system" approach to refinishing. The logic here is simple, basically providing a blueprint for taking an automobile body through the refinishing process. This blueprint provides recommendations on compatible products within the product line from the first layer of primer through to the final application of top coat. From the standpoint of utilizing a proven process in refinishing your Corvette, it's hard to argue with the logic of following the manufacturer's directions from start to finish.

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Even with the caveat of sticking to one manufacturer's line of primers through finish coat, one is left with several choices right from the start. Primary on this list is just what kind of paint to use. Here, the end goal becomes one of the key decision points. Is the project aimed at turning out as true to factory original as humanly possible? Is the goal a show-car finish replicating the factory colors? Or is the project a custom with wild effects, or a budget repaint on a "driver" quality car? The materials in each of these scenarios will generally be quite different. Let's have a little look back at the history of Corvette paints in general and then follow with an overview of the paint systems commonly used today.

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Original Finishes
Up until 1957, Corvettes were painted with a formula of paints referred to as nitrocellulose lacquers. These paints, introduced in the mid-'20s, revolutionized automotive refinishing by providing manufacturers with a quick-drying and relatively stable paint system that could be readily manufactured in a wide array of colors. The name nitrocellulose lacquer is derived from the origin of the resin used in its manufacture, a product of the nitration of cellulose materials, primarily cotton. Lacquer paints are purely solvent-borne, and cure only by the evaporation of these aggressive and volatile solvents. Along with a very fast drying component, the nitrocellulose lacquers offered ease of application, much better color retention than previous paints, and a surface that could be polished to a very high luster. The clincher was that the nitrocellulose paints were ideal for application by spray technique, leading to the early universal use of spray techniques at the manufacturer level.

These days, the original nitrocellulose paint formulations are nearly completely gone. These antiquated paints were definitely a leap forward in the '20s, but lack the durability and resilience demanded by consumers. While it is still possible to obtain nitrocellulose from specialty antique materials suppliers, from a commercial refinishing standpoint, this is an extinct type of paint product.

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GM abandoned nitrocellulose in the late-'50s, turning to acrylic lacquer as a more modern formulation of lacquer. These acrylic lacquers replaced the nitrocellulose resins with a synthetic polymer resin derived from acrylic acid. Although these acrylic resins are also used in enamels, GM utilized the acrylic lacquer paint formulations, which, like nitrocellulose, featured very fast drying times and very easy application. Acrylic lacquer was the primary paint formulation used by General Motors for the majority of the production years in which classic Corvettes were manufactured, serving until the early-'80s. These paints were reasonably durable for the time, and moderately glossy, though the finish would respond exceptionally well to polishing.

The drawback to the factory acrylic lacquer was definitely long-term durability, especially in comparison to today's materials. The paint was susceptible to chemical and environmental damage, fade, and loss of gloss over time. Worst of all, the paint became brittle with age and would crack and check, literally self-destructing. Although acrylic lacquers are still available, these paints are all but obsolete in the world of commercial automotive refinishing. About the only application where these products would be considered would be in contemplating an authentic OEM restoration.

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Eventually, by the '80s, General Motors phased out the lacquer paints in favor of an evolution of modern paint materials, beginning with acrylic enamels, and leading to the basecoat/clearcoat material formulations in use today.

Modern Materials
The basis for the modern commercial finishing processes today are acrylic enamels, urethanes, and increasingly, water-borne or water-based products. As noted, acrylic enamels have been utilized by the OEM manufacturers for many decades, and these paints have long been the standard for the refinishing industry as well. The common air-dry enamels are at the bottom rung of the acrylic enamel ladder, typically the material of choice for a low-dollar, blow-over paint shop. Air dry enamels are very slow to cure, are only moderately tough in terms of chemical resistance, polish poorly, and are prone to fade.

As early as the '70s, highly developed catalyzed acrylic enamel formulations began to come on the market, with cross-linked chemistry that offered exceptional gloss, durability, and an excellent ability to be polished. With these very desirable attributes, the catalyzed acrylic enamels became the mainstay of the commercial and custom painting industry. These paints are still readily available, and offer outstanding value and results. For a single-stage paint application (no clear coat) to mimic the OEM lacquer paint, these catalyzed acrylic enamels are a viable alternative.

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The next level in the evolution of paints was the move from acrylic enamels to urethane paints. These formulations essentially took the best attributes of the catalyzed acrylic enamels and went one step further in durability. Most commercially available paint product lines, particularly the higher-end products, are all urethane formulations.

Paint product formulations are continuing to evolve, with many of the changes having to do with reducing toxicity and meeting environmental concerns. With this there is a move towards reducing solvents, particularly those classified as Volatile Organic Chemicals. VOC levels are closely regulated in the commercial refinishing industry in some areas, prompting paint manufacturers to revise formulations to meet the ever-tightening requirements. An evolution resulting from these forces is the expanding use of water-borne or water-based paint products. These products are designed with formulations to allow them to be soluble in water rather than petroleum-based solvents. While early on, these formulations had significant problems, ongoing development has led to much-improved and sometimes superior products that fit under this category. Some localities have mandated these formulations to replace solvent based paints in commercial refinishing.

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Single vs. Two-Stage
With the advent of modern paint systems, so came the prevalent use of clear coats as the final top coat of the paint system. In older refinishing systems, the paint was what you got-and that was it. The color coats of paint carried all of the color, sheen, and gloss of the finish, a system now referred to as a "single-stage" paint system. This is the type of paint finish that was original to all older production cars, including classic Corvettes.

Clear coats can add to the durability, environmental and chemical resistance, and brilliance of shine in paint's finish. Further, clear can typically be fine-sanded and machine polished for a nearly perfect and glass-smooth surface. With those characteristics recommending a clear-coat finish, it's not hard to see why the use of clear has become almost universal in high-end custom paintwork. Modern clear coats developed back in the '70s, about the same time as the modern catalyzed acrylic enamels and polyurethanes (DuPont Imron) came on the scene. Using a similar catalyzed clear formulation, custom painters found they could achieve previously unimaginable levels of gloss and depth by applying clear over the catalyzed acrylic enamel or polyurethane paints. These early two-stage clear-coat paint systems were applied wet-on-wet, where the high-gloss catalyzed acrylic enamel was sprayed as normal, allowed to solvent flash, and then followed with the clear coats.

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Later, the rather cumbersome early two-stage system developed into using a flat, low-gloss, and fast-drying basecoat followed by the clear, rather than starting with the high-gloss catalyzed full-gloss paint. This is the basis of the popular base/clear two-stage systems that are the mainstay of automotive refinishing today. With the base/clear system, the base contains the pigments and color properties only, and the product sprays on with a very quick dry time, much like a lacquer. All of the gloss and translucence are properties of the clear coat. With this system, the easy-to-apply base color coat minimizes the potential for problems or flaws in the color application, while the clear can be burnished and buffed to provide a spectacular, show-car sheen.

Which is the best approach in refinishing your Corvette-single- or two-stage? It all depends upon the goals of the paint project. For a true OEM restoration, the two-stage option is definitely out. Even if trying to "authentically" duplicate a representation of the OEM finish using modern materials, a single-stage paint approach is more authentic. However, if the goal is a mirror-smooth surface with paintwork that "pops" like no factory machine ever did, a clear-coat paint approach is normally the correct choice. Some painters, however, will still select a single-stage paint approach when refinishing in a solid color, particularly black or even white. With modern single-stage urethanes, solid colors can be color-sanded and polished to a mirror finish as though they are clear-coated, and some consider the clarity of these colors to be superior using the single-stage technique.

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No matter which material you select for your Corvette refinishing project, whether an authentic acrylic lacquer OEM restoration or a gleaming custom base/clear urethane approach, select a high-quality manufacturer for the material and follow the recommendations. All of the major paint manufacturers will carry a full line of primer and undercoat products, often providing a system to take the surface from fiberglass to fully finished. Included here will be the primers used for adhesion, filling, and sealing the surface before paint. Manufacturers will often publish this product data on the material specifications sheet for the topcoat system being considered. Read up on the technical information directly from the paint manufacturers, and solicit some helpful advice from your professional paint retailer or supplier.



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