What Is A Drop Coat In Automotive Painting When it comes to applying automotive coatings, spraying might not be the optimal method for minimizing surface defects, but it remains the sole way to achieve the desired gloss, color effects, and overall appearance that car buyers demand. This article delves into the various defects that automotive coatings can encounter during the application process. While craters stand as the most notorious and panic-inducing among these defects, a range of other issues can also arise, all of which impact appearance and, in some cases, the protective qualities of the coatings. Effectively addressing defects involves preventing them from occurring in the first place.
Defects Related to Surface Tension Numerous common defects seen on car surfaces stem from surface tension dynamics. These include craters, dewetting, telegraphing, picture framing (also known as fat edges), and poor edge coverage.
Almost everyone familiar with coatings has come across craters, though their causes might not be so well-known. Craters result from low surface tension contaminants present either on the painted substrate, within the paint itself, or landing on the paint surface. This creates a surface tension gradient that leads to material flow away from the low-tension region, forming circular depressions. While minor craters can sometimes be polished away, deeper ones require sanding and repainting. Their origins can vary, often involving contaminants falling onto the wet coating during or shortly after application. Identifying the contaminant and its source is a challenge due to multiple possible causes.
Dewetting occurs when a coating initially wets a surface but subsequently retracts, leaving the film uneven or forming beads, islands, pinholes, or craters. This problem often arises due to dirty or contaminated surfaces. Thorough cleaning and pretreatment of metal and plastic surfaces are crucial. Various tests, such as measuring contact angles, can determine surface wetting characteristics, but simpler methods like cotton swab and marking pen tests can also provide insights.
This defect reproduces surface features from an undercoat or substrate onto the applied coating. It is primarily caused by surface tension-driven flow, leading to material movement away from residues or sharp edges, resulting in streaks or distinct patterns.
Picture Framing and Poor Edge Coverage:
These issues stem from surface tension gradients during baking, causing material flow towards edges or away from them. Picture framing often occurs along edges and results from evaporating solvents more rapidly from those regions, causing a surface tension imbalance.
Defects from Volatile Compounds Volatile-related defects encompass a set of problems originating from solvents that need to evaporate during the flash or bake stages. These defects include popping, gassing, and air entrapment.
This defect arises from trapped solvents or other volatiles escaping during baking, forming pinholes, craters, or bubbles. Solvent popping can be difficult to distinguish from substrate gassing or air entrapment. Waterborne basecoats can introduce new sources of popping due to dehydration effects.
Gassing defects occur when volatiles originating from the substrate migrate into the paint layer during baking, causing pinholes, bubbles, or other disruptions. It is common over plastic substrates and zinc-coated steel, often due to air or moisture escaping voids in the substrate.
Bubbles trapped in the paint during manufacture or application can lead to bubbles, pinholes, or crater-like defects in the cured film. Air entrapment can result from mixing and stirring processes during manufacturing or application.
While defects in automotive coatings can mar appearance and functionality, they can often be prevented or reduced through careful control of surface tension, rheology, and maintaining cleanliness in both paint plants and auto factories. Identifying defects and their causes demands expertise and specialized tools. Microscopic examination, cross-sectioning, and techniques like scanning electron microscopy and Fourier Transform IR can be essential in root cause analysis.