In stark contrast to today, not only were cars available in a technical range of colors and shades, but they also utilized different types of paint to perform different functions.
This is great for the end user, but when it comes to automotive paint coatings, each coating type has its own challenges. Today's customers expect the highest quality, so these challenges need to be understood and mastered in order to achieve a good surface finish - before automotive paint finishers find quality issues that affect their bottom line.
car paint structure
A common automotive finish consists of three distinct layers: base coat, base coat, and clear coat. Each of these plays a specific role in the overall coating structure, and each of these can be a completely different chemistry than the other coatings, requiring different considerations during application and curing. For the sake of clarity, we will ignore the pretreatment process that occurs before the decorative paint operation.
Primer
A primer is the initial paint layer used to prepare an individual component of an automotive body or primer. It smooths out tiny defects in the manufacturing process and enables the undercoat to bond properly by creating an adhesive layer. We're all familiar with the common gray or red, but there are also "color key" primers, designed to support and/or accentuate an undertone. Primers are available for virtually all solvent- and water-based base chemistries, including epoxies, polyurethanes, polyesters, acrylics, and more. Some manufacturers even use powder coatings as the primary coating, but for the purposes of this discussion we will focus on liquid primers.
The primer is the thinnest of the decorative layers and therefore has the greatest potential for uneven filming problems. Also, as an infrastructure, it provides the foundation upon which other layers are built. It is easy to understand how uneven and continuous primer layers can transmit these imperfections through the base coat and clear coat, thereby destroying the aesthetic quality of the final top coat. After all, if you paint on sandpaper, you expect the same smooth, glossy finish that you can paint on glass?
When priming, paint temperature control is very important because temperature directly affects the viscosity of the paint, which in turn affects the flow and integrity of the final primed surface.
Primer
Primer is probably the paint layer that most people think of when they think about car paint. But in reality, the primary function of a primer is to provide color and other visual effects, such as a metallic or "glitter" look, or pearlescent or iridescent colors. With modern formulations, it is even possible to "shift" the color (e.g. from red to blue) depending on the angle at which the surface is viewed!
Like its primer counterparts, primers are available in a variety of water and solvent-based chemistries, each requiring its own consideration for great performance. For primers, film thickness is more important than the other two coats because this is where the color is controlled. Thinner films produce different colors than thicker films due to the structure of the pigments and metal flakes. Therefore, controlling the viscosity to maintain a consistent and repeatable film configuration is essential. Due to its relationship to viscosity, inconsistent paint temperatures can result in inconsistent color.
Who knew the temperature of the paint when it was painted could determine the color of a car?
varnish
A clear coat is the final protective layer applied to a car or its components. It's important to remember that varnish is still paint - it's just paint without any pigments. What varnish does is protect from the elements—not just the weather, but dirt, UV rays, scratches, and anything else the world might care about. As a protective layer, it is also the thickest of the three layers. Film changes across the part are common as more coating bleeds and cures, these can have an effect on gloss, orange peel, etc.
Without controlling the clear coat temperature (and thus viscosity), it can become too thin and not properly protect the underlying base coat, or too thick, thus using too much material and compromising the bottom line of automotive paint .
