In the realm of spraying automotive coatings, the worst possible way to apply them is without a focus on the minimization of surface defects, which are essential for achieving the gloss and color effects that car buyers demand. This article surveys the various defects that I’ve seen in my experience when coatings are applied incorrectly. Craters are infamous for causing the greatest panic in an auto plant, but a number of other defects can also occur, which not only hurt the appearance but also interfere with the corrosion and weathering protection aspects of the coatings. Repairing these defects often can lead to more problems, making the best strategy to prevent them from occurring in the first place. Understanding and addressing these application issues is key to maintaining the integrity and aesthetics of automotive finishes.
Automotive Paint Spot Repair
In the fast-paced world of automotive bodywork, “Automotive Paint Spot Repair” has emerged as a game-changer, revolutionizing efficiency and productivity. This specialized technique focuses on repairing small damages, such as dents, scratches, and chips, confined to a limited area rather than repainting entire parts. Emphasizing speed, precision, and cost-effectiveness, spot repair not only reduces material consumption and working time but also enhances a bodyshop’s capacity to handle multiple projects simultaneously. Ideal for a range of finishes and types of damage, this method is instrumental in boosting profitability, reducing energy consumption, and offering quick turnaround times – all while ensuring high customer satisfaction. Mastering these spot repair processes is not just about repairing a car; it’s about elevating the standards and reputation of your bodyshop in today’s competitive market.
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Surface Tension-Related Defects
In the automotive industry, surface tension-driven defects are among the most common day-to-day defects observed on car bodies. These include issues like craters, dewetting, telegraphing, picture framing, and fat edges, as well as poor edge coverage. Coatings people are often familiar with these defects but may not know much about their causes and remedies. These defects are caused by low surface tension contamination on the substrate being painted or when contamination falls on the paint. This leads to a surface tension gradient, causing the paint to flow towards the low surface tension area, resulting in a circular low spot.
Shallow craters can sometimes be polished out, but deeper ones often require sanding and repainting. The size and appearance of these defects can vary depending on the given contaminant. In the majority of cases encountered in auto plants, these contaminants fall into the wet coating during or soon after application. Identifying the contaminants and their source can be difficult. Rigorous inspections in paint shops are essential to identify sources such as poor substrate cleaning, oily overhead chains, smoking ovens, oil in compressed air, or dirty paint booths.
Crawling is another defect often seen on dirty or contaminated surfaces. It’s critical to ensure good cleaning and pretreatment of metals through methods like power washing or solvent wiping and plastic parts to prevent such defects. Additionally, defects can reproduce the surface features of the undercoat or substrate over which the coating is applied. The main cause is the flow of the coating material due to surface tension. Such problems caused by surface tension-driven flow highlight the importance of thorough preparation and cleanliness in automotive painting processes.
In automotive painting, surface tension-related defects arise when solvents evaporate rapidly, especially at the edges of painted areas. Regions richer in higher surface tension coating vehicle tend to experience a surface tension gradient, causing the flow of material towards the edges. A common issue is the bead that forms at the back edge of a painted panel. When the edge is heated up faster during baking than the rest of the part or panel, it initially lowers the surface tension gradient and causes flow away from the edge, producing an offset bead. This is a different, related defect where the material flows away from sharp edges during the bake.
Surface tension forces work to minimize the surface area of the coating, which can be problematic, especially with ED primers that are supposed to provide good edge coverage. During the deposition and flow in the baking process, the coating can become thin or provide no coverage on sharp edges. To mitigate this, silicone surfactants are effective when used in very low levels. However, silicone oils should be kept away from paint in paint plants and auto plants. Polyether, polyester, modified polydimethyl, polymethyl, and alkyl siloxane surfactants are often added at 0.1-0.3% of the total paint to address this issue.
Moreover, low surface tension solvents such as butanols, 2-ethyl hexanol, and VM&P Naphtha are used to lower surface tensions in liquid coatings. The strategy here is to raise the low shear viscosity of the paint. By developing a combination of surface tension control and rheology optimization, many of these defects can be minimized, ensuring a smoother and more consistent finish on automotive surfaces.
Other Flow Defects
In the spectrum of automotive paint defects, apart from surface tension driven flows, there are issues related to gravity driven flow such as sagging, lack of flow and leveling, and orange peel. For instance, too much flow, often caused by gravity driven flow on vertical surfaces, leads to Automotive topcoat sags. While these sags are less common in ED primers, they can still occur due to run-out of paint trapped in seams, flanges, or inner areas. During baking, this excess paint boils and runs out, producing noticeable drips. These defects can be smoothed out by sanding before the basecoat is applied.
Sags in automotive topcoats can be subtle. The spray application of paint produces a pattern of droplets, and bumps can form from small sags or microsags, particularly on vertical surfaces. These sags tend to flow out on horizontal surfaces. As a result, verticals on automobiles often look rough in comparison to hoods, roofs, and deck lid horizontals. Auto companies are increasingly concerned about sagging when paint is applied to verticals, often opting for paint with higher viscosity.
Cold sag occurs upon application, while hot sag happens in the oven. These can be reduced or prevented by raising the low shear rate viscosity of the paint, applying thinner coats, or increasing film viscosity with faster solvents, thickeners, or thixotropes. The term orange peel refers to a bumpy coating surface. What’s considered excessive is often in the eye of the beholder; a Quality Supervisor might find some paint jobs acceptable that others deem to have too much orange peel, often due to poor application techniques.
Foreign Material (Dirt)
In the world of automotive paint defects, foreign material or dirt is one of the most common causes. My experience in automotive coating has shown that dirt in paint films can come from various sources: fibers, sanding dust, metal particles, oven dirt, general dust, and grit. In many cases, as shown in Figure 6, a fiber is the most common type of coatings dirt. Dirt defects often appear as bumps, and when cross-sectioned, they give an idea of what is in the bump. Not only do these include typical dirt elements, but also resin gel particles, pigment agglomerates, paint chips, and flakes, all of which are considered dirt.
Additionally, there are other defects that can resemble dirt, such as paint drops, gun spits, and overspray. Substrate defects like solvent pops and gassing are often mistaken for dirt. When dirt is found on auto coating surfaces, it often leads to sanding and repainting. High bake repair painting involves running the body through the oven again, which can decrease chip resistance. For repairs on finished bodies, a low bake repair can be carried out using a catalyzed version of the clearcoat, applied and then baked with a heat lamp or heat gun. This surface can have poor scratch resistance, and larger areas are often suffering from mar and scratch problems in the field.
It’s a misconception that paint is often to blame for dirt problems; it’s rarely the culprit. Instead, dirt in an auto plant is typically due to the painting process, poor work practices, or bad air filtration. Effective management of these factors is crucial in reducing the incidence of dirt-related paint defects.
Volatile-Related Defects
Volatile-related defects in automotive coatings arise mainly from the evaporation of solvents in the paint. These defects manifest as solvent pops and pinholes, where solvents trapped beneath the paint’s surface expand during the baking process and burst through the paint film, leaving behind small craters or pops. These defects are particularly prevalent in coatings with high solvent content or when applied in thick layers. In my experience, the baking temperature and time play crucial roles in the emergence of these defects. If the baking process is rushed or the temperature is too high, solvents do not have enough time to evaporate slowly and evenly, resulting in a poor finish.
Another form of volatile-related defect is gassing, which occurs when air or gases are trapped within the paint layer and expand under the heat of the baking oven. This expansion causes bubbles or blisters in the paint film, leading to a blemished surface. The quality of the paint, application technique, and environmental conditions during painting, such as humidity and temperature, significantly influence the occurrence of these defects. Proper ventilation, controlled environment, and adequate flash-off time between coats are critical in preventing gassing.
Finally, micro-bubbling or micro-foaming can occur when very fine gas bubbles are trapped in the paint film. Unlike larger bubbles caused by gassing, these micro-bubbles are often not visible to the naked eye but can affect the gloss and uniformity of the paint. Adjusting the paint formulation, reducing the thickness of the applied layers, and ensuring optimal baking conditions can help mitigate these volatile-related defects. Understanding and controlling the evaporation rate of solvents is key to preventing these issues and achieving a smooth, flawless finish in automotive painting.
Swirl Marks
Swirl marks are a common form of new car paint defects, often caused by improper washing techniques. These marks are typically the result of washing with an abrasive or dirty mitt, or sometimes due to the brushes in an automatic car wash. The good news is that paint correction can either completely remove these marks or greatly reduce their visibility through careful polishing. The approach to correcting these marks depends largely on the severity of the swirling. In my experience, ensuring proper washing techniques and using the right materials can significantly minimize the occurrence of swirl marks on automotive paint surfaces.
Minor Scratches & Scuffs
Among automotive paint defects, small scratches and scuffs are some of the easiest to remove through paint correction. These car paint defects can be caused by a variety of factors, ranging from accidental keys scrapes, gravel strikes, brushing against bushes, or even fingernails dragging across the surface. Fortunately, in most cases, paint correction can help, as these marks can often be buffed out, leaving the paint looking smoother and rejuvenated. The effectiveness of this correction process largely depends on the depth and severity of the scratches or scuffs, but for minor blemishes, it typically yields excellent results, restoring the vehicle’s aesthetic appeal.
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Deep Scratches
A deep scratch that penetrates beyond the clear coat and into the paint or even the metal surface of a vehicle presents a significant challenge. The level of damage from such a scratch means it cannot be repaired with standard paint correction methods and requires more advanced techniques. Depending on the severity and depth, options range from paint touch-up to full panel repainting. These methods are more intensive and necessitate a precise color match and careful application to seamlessly blend with the existing paintwork, restoring the vehicle’s appearance effectively. Deep scratches not only affect the aesthetic but can also lead to further deterioration of the vehicle’s body if left unaddressed.
Water Spots
Water spots are a prevalent form of car paint defects, typically caused by mineral buildup from hard water. These spots often occur when a vehicle is not dried well enough after a wash or from getting hit by sprinklers, with the latter being a common culprit. To eliminate these water spots, especially the more recent ones, water spot remover products can be effective. However, the longer a spot has been on the paint, the harder it becomes to remove. Paint correction techniques may be able to help, depending on the severity of the mineral deposit and how long it’s been left on the paint. The process of correction is often able to remove or reduce water spot marks, although it’s not fully guaranteed. This is especially true for older or more stubborn water spots, where the minerals have had more time to bond with the paint surface.
Etching from Bird Droppings
The acidity in bird droppings can burn through automotive paint if left on the surface for too long, leading to etching. This etching primarily affects the clear coat and can sometimes be leveled by performing paint correction. However, if the etching has gone too deep, mere paint correction may not be sufficient. In such cases, more intensive treatments or even repainting might be necessary. Regular cleaning and immediate removal of bird droppings can significantly minimize the risk of such damage, preserving the integrity and appearance of the car’s paintwork.
Oxidation
Car paint oxidation is a common issue that occurs when a vehicle’s paint is exposed to the elements, with the main culprit being UV rays. This exposure can cause the paint to become dull and faded. If there is enough clear coat still intact, paint correction can often help bring back the shine. Regular maintenance, including waxing and using UV protection products, can significantly slow down the oxidation process, preserving the paint’s vibrancy and longevity. It’s crucial to address oxidation early, as prolonged neglect can lead to irreversible damage, requiring more extensive repairs or repainting.
Heavy Oxidation and Clear Coat Failur
Heavy oxidation, when present alongside severely damaged clear coat, reaches a point where paint correction cannot repair the damage. In cases of clear coat failure, characterized by peeling, flaking, or blistering, the damage is often too extensive. In such a scenario, repainting becomes the only viable option to restore the vehicle’s appearance. These defects usually occur due to prolonged exposure to harsh environmental conditions without adequate protection or maintenance. Addressing clear coat issues early can prevent them from escalating to a stage where extensive and costly repainting is necessary.
