Coatings are used on buildings and structures to protect the components of each from weather, and any conditions the components will be exposed to. A properly applied coating can last for years, but a coating that has improperly applied can fail quickly. Most coating failures are caused by insufficient surface preparation, which lead to a failure in the bond of the coating to the substrate. Other common causes of failure include inadequate temperature during coating, excessive moisture or air movement, improper mixing of components, and inadequate or excessive thickness of the applied coating.
Poor surface preparation accounts for up to 75% of coating failures. This includes improper cleaning of the surface of contaminants, as well as inadequate surface profile. Contaminants can include water-soluble salts, dust from abrasive blasting of the substrate surface, as well as environmental dirt and dust that was not properly cleaned off the surface. Surface profile is important because it allows the coating to adhere to the substrate surface.
Coatings applied with poor surface preparation will peel, and in some cases blister. Poor surface preparation can be shown by the following:
- Adhesion would be weaker at the coating interface with the poor surface preparation.
- The underside of the coating that is failing may reveal dust and debris
- Surface profile measured and shown to be less than the manufacturer’s specification
Environmental Conditions during coating application
The conditions during coating application and curing are important for a successful application. A manufacturer’s coating specification will have the criteria that need to be met for a successful coating. This will include an appropriate temperature range, an appropriate relative humidity, and how high above the dew point the temperature of the surface being coated should be.
Coatings applied below the required temperature range may cause the coating to be soft or tacky after curing. Too high of an application temperature can causes the surface of the coating to dry too quickly and form a film on the coating surface. The coating behind the surface coating then loses solvent to evaporation which can causes checking, alligatoring, and wrinkling of the coating.
Coating specifications typically specify a maximum relative humidity for application. A coating applied above the maximum RH specified may not cure properly because the rate of solvent evaporation is decreased in a high humidity environment. Coatings applied in high humidity conditions may be soft or tacky after curing, and they may cause the coating to run or sag because of the delayed solvent evaporation. Coatings applied in high humidity may also have a milky appearance and loss of gloss. Low humidity can contribute to dry spray/overspray occuring. Dry spray is where the solvent in the paint droplets evaporates before the coating reaches the substrate surface. This doesn’t allow the paint droplets to spread out on the substrate surface, causing a rough, sandy appearance.
It is necessary for coatings placed in exterior environments to have resistance to UV light exposure. Molecules in the coating absorb the UV radiation from the sun and over time this radiation breaks the molecular bonds of the coating binder, causing the coating to degrade. This degradation usually materializes as chalking on the coating surface. Chalking is a powdery film on the surface of the coating that is the result of the coating binder degrading, and can be evaluated using ASTM D4214, Standard Test Methods for Evaluating Degree of Chalking on Exterior Paint Films.
Some coatings are two part systems which require mixing of the two components on the job-site. Improper mixing of these components can lead to coating failures. Improper mixing of a coating system includes not using the appropriate ratio of each component, as well as not mixing thoroughly.
Coatings that have been improperly mixed can change the final properties of the coating, making it unsuitable for it’s intended purpose. Coatings not mixed at the proper component ratios may exhibit:
- Softer film formation
- Bubbles trapped under the surface
- Coating that does not cure
- Slower curing of coating
Coatings that have been improperly mixed can be shown by:
- Solvent sensitivity testing using the Solvent Rub Test described in ASTM D5402
- Carbon, hydrogen, nitrogen ratios – Comparing the ratios of nitrogen, carbon, and hydrogen of a properly mixed coating sample, and the coating from the job-site to see if the ratios are similar. Different ratios would indicate the mixing ratios of the coating are out of tolerance
- FTIR (Fourier-transform Infrared spectroscopy) – Comparing the infrared spectrum of a properly mixed coating, and the coating from the job-site, to see if peak intensities relative to other peaks are similar, which would indicate the coating compositions are consistent
Coatings that have been applied at thicknesses outside the manufacturer’s specification can lead to premature failure of the coating. A coating that is too thick or thin will often not have the same physical properties as a coating applied within the correct thickness range.
Coatings applied too thin can cause pinpoint rusting, because there is insufficient barrier from the elements. A coating that is too thin may also be less flexible and be prone to cracking.
Excessive coating thickness causes the coating to dry and cure slower, which can lead to many problems. In the initial curing and drying, the coating can sag and run. Excessive thickness can also cause the coating to crack because there are increased internal stresses on the coating as the coating increases in thickness. This is especially important for coatings that are more brittle.
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