The manufacturer's Product Data Sheet (PDS) should always be consulted when recommending a paint or paint system for specification, or when responsible for the application of a paint. PDSs generally contain information on appropriate use and environment, performance characteristics, and application information including mixing, application methods and equipment, and suggested thickness ranges for materials. The information provided by the manufacturer should be followed as closely as possible to better improve the performance of the paint. In this article, we examine possible failure mechanisms that can occur if the coating thickness is insufficient or excessive.
Consider the target coating thickness when selecting a coating. The manufacturer's recommended range and the thickness indicated in the project specification should match. If discrepancies exist, the specifier and paint manufacturer should be contacted for resolution. When applied at the proper thickness, the physical and protective properties of the coating material are optimized. If sufficient attention is not paid to the coating thickness of the material, and the final product is coated with insufficient or excessive coating thickness, coating performance problems often arise.
Insufficient coating thickness
There are many reasons for undercoating, but basically coating thickness was not specified or communicated correctly, underapplication or the volume solids content of the coating was not accurately reported. The quality of paint should not be judged by the appearance of the covering. Namely, it looks good enough. Despite the cosmetic advantages of coatings, the goal is usually to protect the substrate from corrosion or other environmental factors. If the coating is applied until complete coverage appears without regard to the thickness specified by the manufacturer, the final dry film thickness may be lower than the target thickness to achieve good coating performance in the particular environment.
When a specified coating thickness range represents thinner coating films (eg, 1.0-1.5 mils), the application importance of being within that range is magnified. If the coating is applied at a thickness of 0.5 mil, the film thickness will be reduced by 32 – 50%. Special care is required when handling the film to ensure proper thickness is obtained. In contrast, if the specified range is 20 to 30 mils and the actual film thickness is 19 mils, a 5% reduction in coating thickness may not significantly affect the performance of the coating material.
When coating films are applied well below the recommended thickness range, several problems may arise:
Substrate Visibility
When proper coverage cannot be achieved, the substrate or underlying coating may become visible through the paint. While this is not a desirable aesthetic condition, it can also leave the underlying coating material or substrate vulnerable to corrosion, degradation from exposure to solar radiation (sunlight) or other environmental influences. For example, if a polyurethane topcoat is applied over an epoxy coating and there are several sparse areas, the epoxy coating is more prone to chalking due to accidental exposure to UV light. Results include color and gloss spots, uneven corrosion patterns, and less time until repair painting is required.
precise rust
Accurate rusting can occur when the film does not provide an adequate barrier or cathodic protection to the underlying metal substrate. For example, any primer, including zinc-rich primers applied to blast-cleaned steel surfaces where the surface profile depth is greater than the coating thickness, will not adequately cover the tips of the anchor profile peaks (especially rouge peaks), leaving Steel heads with little or no protection, which may stick to the coating. Failure to stir zinc-rich paint during construction can exacerbate the problem by not providing enough zinc material to effectively protect the steel from corrosion. The least protected parts will rust.
Cracking/brittleness of the coating
When the coating film is applied below the target thickness range, the physical properties of the coating film itself may be compromised. For example, a modified polyurethane liner, which has inherently high cohesive strength at a thickness of 30 mils, will be less brittle than an applied thickness of 5 to 8 mils. A decrease in coating flexibility corresponds to a decrease in the cohesive strength of the film and can lead to cracking or delamination.
Correcting insufficient film thickness is not always easier than adding more or making the next coat thicker. Inorganic zinc coatings generally have poor intercoat adhesion when used in multiple layers. Anti-corrosion epoxy primers need to consider the drying and curing interval time, the loss of primer thickness also reduces the function of the primer - may not make up for the next thicker coat. Topcoats may require professional scraping before additional coats can be applied.
Clearly, owners do not want to deal with the consequences of poor film performance. Contractors certainly don't want to deal with low film thickness. Two good arguments for achieving the required minimum film thickness. The other side of the coin failed to stay below the maximum coating thickness. This also has consequences.
Coating thickness is too large
As with underthickness, the causes of overcoating are application-related, but the causes of application errors are variable. Often, applicators are not aware of the effects of excessive coating thickness on coating performance. Protective coatings are designed to function within a specific thickness range based on the chemistry and intended use of the coating. Therefore, the more protective coating, the better. In fact, some additional problems or failure modes were attributed to excessive coating thickness rather than excessive coating thickness due to insufficient thickness. Several consequences of excessive coating thickness are discussed below.
sagging
Wet paint can run or sag on vertical surfaces if too much paint is applied. Less viscous paints are especially prone to sagging and sagging if applied in excess thickness. When the film sags, we are left with an uneven film surface. Not only does this create too much film in the sag area, but it can also lead to insufficient surface thickness and the aforementioned problems.
cracking/delamination
Certain coatings, such as epoxies, are prone to cracking and delamination when thickened beyond the manufacturer's recommended range. The high epoxy functionality of these resins forms a highly crosslinked polymer network post cure that exhibits high temperature and chemical resistance but low flexibility. Additional internal stresses associated with excessive coating thicknesses often lead to material cracking, which can subsequently lead to delamination of the coating. Typically, delamination occurs at the weakest interface. If the coating is too thick, it usually reduces the cohesive strength of the material. Cohesive failure of coatings applied too thick is not uncommon in cross-linked coatings. in addition,
wrinkled
In the case of excessive coating thickness, the top surface of the coating may dry out or cure before the coating becomes large. In these cases, the top surface develops a skin of dry material at rest on top of the soft, uncured soft material, which can cause the surface to deform when the underlying material is in motion. The ridges and valleys of the skin are seen as wrinkles. Alkyd coatings are especially susceptible to surface wrinkling when overused because they cure in the atmosphere through exposure to oxygen. The oxidation process will occur first at the surface and throughout the thickness of the coating, but at a much slower rate. Problems can arise with films that are applied too thickly so that the majority of the film remains soft and uncured with less exposure to the atmosphere.
increase drying time
Coating drying and curing times are generally directly related to coating thickness. If the coat is too thick, the delayed drying time can affect recoating schedules, solvent release, and buildup of dirt and debris due to abnormally extended soft surfaces. This can result in a poorly coated finish or unwanted appearance.
Improper curing
Excessive film build-up can cause problems during curing, as well as wrinkling and increased drying time. Certain materials may not fully cure before being put into service, resulting in a coated surface that is too soft, easily damaged and unsuitable for use. Solvent entrapment can also occur if the top surface cures but the underlying material does not. Since a thicker film creates a thicker, less permeable barrier, any entrained solvent or moisture will be more difficult to penetrate through the system. This solvent entrapment can lead to blistering of the coating.
