Understanding Liquid Film Defects
Col-Met Engineered Finishing Solutions’ Judith Lietzke explains the causes of paint film defects and how to prevent them.
Q. We’ve recently started experiencing film defects in our liquid coating application. How do I define the cause of the defect and identify precautionary measures that can be taken?
A. You are not alone. Defects happen for various reasons in both electrostatic and non-electrostatic applications. The first thing to understand is that all defects are caused by a breakdown in the painting process. This breakdown could occur within the actual process, or due to an inadequacy in the coating material or substrate. The first step to understand the type of defect, identify the cause and then take precautionary measure to avoid defects in the future.
Types of Film Defects
Blisters are small, raised areas that contain, or once contained, moisture, oil, solvent or grease. With metal, they can be caused by contamination left on the surface before it is painted. With wood, they typically are caused by moisture escaping from within the wood onto its surface. When that moisture under the coating gets warm, it expands, and this exerts enough pressure to raise the film into a blister. To prevent blisters from forming, make sure the surface to be painted is free of contaminants and dry before applying the paint. When working with wood, it’s also important to manage its moisture content through temperature and humidity, as well as identify another exit route for the moisture instead of pushing up through the paint.
Bubbles closely resemble blisters, however, bubbles (as well as craters) once contained solvent vapor rather than moisture. A crater is a small, concave, depressed area formerly covered by a bubble. Its rounded bottom and built up sides are caused by the breaking of the bubble. Bubbles tend to form during a bake cycle when the top layer of paint film skins over before most of the solvent has had time to escape. If, or when, they break, craters are formed.
Bubbles typically only form in baked coatings, not in air-dried finishes. Besides forming due to trapped solvent, bubbles and craters can also be caused by inadequate flash time before the bake cycle, as well as from an extra-heavy wet film application. Other causes could be a solvent blend that evaporates too slowly or an insufficient primer bake, which can leave an excessive amount of solvent in the primer, causing bubble formation in the topcoat during the topcoat bake cycle.
One way to prevent bubbles and craters is to allow enough flash time before a bake cycle. Allow the surface to rest first before baking, and apply the coating in several thin layers instead of in one heavy coat. Also, maintain specified primer bake cycles and use proper solvents.
Color mismatch is when several different batches of the “same” material are applied, yet the colors do not appear the same. Possible causes include variations in the degree of film wetness; inadequate agitation of the material; low film build; different application procedures; different substrates; different surface textures; or overbake. It can be prevented by consistency in paint agitation, degree of film wetness, film thickness and application procedures.
Dirt consists of any and all contaminants, including lint, dust, small clusters and overspray paint debris in or around painted surfaces. Causes of dirt contamination can be related to poor housekeeping, including in part preparation; inadequate facilities; or poor painting practices. The goal should be to eliminate any possible sources of lint or dust, or at least minimize them as much as possible. In addition, spray booth overspray should be kept at a minimum. Adequately stir the paint and use only recommended solvent for reducing paint, and paint should always be filtered prior to use. Keep extracurricular air or booth air at a minimum as well to reduce the risk of floating particles in the spray booth area.
Fisheyes are defects in the paint film that appear as small depressions with a mound in the center that resembles a fish eye, thus the term. Fisheyes are almost always caused by residual oil, grease or silicone products. The only method of prevention is to keep all silicone products out of the paint area.
Gloss variation is differences in the surface’s ability to reflect light. Causes include wet spots in a base coat, insufficient oven makeup air, excessive humidity in the flash zone, insufficient film build or excessive oven temperature. Prevention is as simple as maintaining consistent film thickness or using proper paint application techniques. Also, avoid excessive humidity in flash zones and make certain that oven makeup air is sufficient. Lastly, operate bake ovens at specified temperatures.
Mottling occurs when metallic paint is applied excessively wet and the color pigments separate from the metallic flakes. Those metallic flakes form light regions in the center of dark rings. Mottling can most commonly be tracked back to applying paint too thick or extra wet, which indicates the paint contains excessive solvent. To prevent mottling, do not apply paint too wet, or apply the initial coats at normal wetness and then apply the final wet-on-wet coat somewhat dry.
Orange peel is characterized by repetitive bumps and valleys, similar to the surface of an orange. Causes are related to excessive dry spray, poorly atomized spray, overly thin coating material or rough substrates. To prevent orange peel, do not dry spray, properly atomize the paint, carefully monitor paint thickness and check surface smoothness.
Runs or sags are the result of the downward flow of paint before the curing process hardens the film and stops that flow. Runs or sags can be caused by dirty applicators, buildup on the fluid tip; poor operator technique; the gun being too close to the object being painted; excessive solvent; insufficient air pressure; or inadequate flash time. Prevention is as simple as cleaning the applicator, using the correct amount of solvent, using correct air pressure, providing adequate flash time, warming parts or proper operator training.
Solvent pops, boils and pinholes are all tiny craters on the surface of the paint film. These are very small versions of bubbles and craters. The cause is overly rapid solvent loss from the wet paint. These defects may also be caused by pigment clusters, surface roughness, high or low solvent evaporation rates, high oven temperature, or trapped air. To prevent this type of defect then, avoid pigment clusters, check the surface roughness of the part, control solvent evaporation rates, and use correct oven temperatures.
Soft paint film occurs when the coating does not harden to the designated cure time. This can be related to low oven temperature, low oven air makeup, contaminated softening agents, excessive paint storage, excessive film build or insufficient cure time. Prevention can be as simple as monitoring oven temperature; maintaining oven air makeup; avoiding softening agents such as wax, grease or oil; using proper amounts of retarder solvent; applying proper film build; and curing as specified by the material manufacturer.
Judy Lietzke is marketing manager for Col-Met Engineered Finishing Solutions.