A coating that prematurely reaches the end of its useful life is said to have failed. Even protective coatings properly selected and applied on well-prepared surfaces gradually deteriorate and eventually fail. The speed of deterioration under such conditions is less than when improper painting procedures are earned out. Inspectors and personnel responsible for maintenance painting must recognize signs of deterioration to establish an effective and efficient system of inspection and programmed painting. Repainting at the proper time avoids the problems resulting from painting either too soon or too late. Applying coatings ahead of schedule is costly and eventually results in a heavy buildup that tends to quicken deterioration of the coating. Applying a coating after it is scheduled results in costly surface preparation and may be responsible for damage to the structure, which may then require expensive repairs.
In the following sections, well look at some of the more common types of paint failures, the reasons for such failures, methods of prevention, and cures.
SURFACE PREPARATION FAULTS
Paint failures can result from many causes. Here, well look at some of the most common caused by faults in surface preparation.
Alligatoring (fig. 8-3) refers to a coating pattern that looks like the hide of an alligator. It is caused by uneven expansion and contraction of the undercoat. Alligatoring can have several causes: applying an enamel over an oil primer; painting over bituminous paint, asphalt, pitch, or shellac; and painting over grease or wax.
Peeling (fig. 8-4) results from inadequate bonding of the topcoat with the undercoat or the underlying surface. It is nearly always caused by inadequate surface preparation. A topcoat peels when applied to a wet, dirty, oily or waxy, or glossy surface. All glossy surfaces must be sanded before painting. Also, the use of incompatible paints can cause the loss of adhesion. The stresses in the hardening film can then cause the two coatings to separate and the topcoat to flake and peel.
Blistering is caused by the development of gas or liquid pressure under the paint. Examples are shown in figure 8-5. The root cause of most blistering, other than that caused by excessive heat, is inadequate ventilation plus some structural defect allowing moisture to accumulate under the paint. A prime source of this problem, therefore, is the use of essentially porous major construction materials that allow moisture to pass through. Insufficient drying time between coats is another prime reason for blistering. All blisters should be scraped off, the paint edges feathered with sandpaper, and the bare places primed before the blistered area is repainted.
A coat of paint is dry when it ceases to be "tacky" to the touch. Prolonged tackiness indicates excessively slow drying. This may be caused by insufficient drier in the paint, a low-quality vehicle in the paint, applying the paint too thickly, painting over an undercoat that is not thoroughly dry, painting over a waxy, oily, or greasy surface, or painting in damp weather.
Sometimes a glossy paint fails to attain the normal amount of gloss. This may be caused by inadequate surface preparation, application over an undercoat that is not thoroughly dry, or application in cold or damp weather.
One particular area you, as a Builder, have direct control over is application. It takes a lot of practice, but you should be able to eliminate the two most common types of application defects: crawling and wrinkling.
Crawling (fig. 8-6) is the failure of a new coat of paint to wet and form a continuous film over the preceding coat. This often happens when latex paint is applied over high-gloss enamel or when paints are applied on concrete or masonry treated with a silicone water repellent.
When coatings are applied too thickly, especially in cold weather, the surface of the coat dries to a skin over a layer of undried paint underneath. This usually causes wrinkling (fig. 8-7). Wrinkling can be avoided in brush painting or roller painting by brushing or rolling each coat of paint as thinly as possible. In spray painting, you can avoid wrinkling by keeping the gun in constant motion over the surface whenever the trigger is down.
Not all painting defects are caused by the individual doing the job. It sometimes happens that the coating itself is at fault. Chalking, checking, and cracking are the most common types of product defects you will notice in your work as a Builder.
Chalking (fig. 8-8) is the result of paint weathering at the surface of the coating. The vehicle is broken down by sunlight and other destructive forces, leaving behind loose, powdery pigment that can easily be rubbed off with the finger. Chalking takes place rapidly with soft paints, such as those based on linseed oil. Chalking is most rapid in areas exposed to sunshine. In the Northern Hemisphere, for example, chalking is most rapid on the south side of a building. On the other hand, little chalking takes place in areas protected from sunshine and rain, such as under eaves or overhangs. Controlled chalking can be an asset, especially in white paints where it acts as a self-cleaning process and helps to keep the surface clean and white. The gradual wearing away reduces the thickness of the coating, thus allowing continuous repainting without making the coating too thick for satisfactory service.
Figure 8-8.Degrees of chalk.
Do not use a chalking or self-cleaning paint above natural brick or other porous masonry surfaces. The chalking will wash down and stain or discolor these areas.
Chalked paints are generally easier to repaint since the underlying paint is in good condition and requires little surface preparation. But, this is not the case with water-thinned paints; they adhere poorly to chalky surfaces.
Checking and Cracking
Checking and cracking are breaks in a coating formed as the paint becomes hard and brittle. Temperature changes cause the substrate and overlying paint to expand and contract. As the paint becomes hard, it gradually loses its ability to expand without breaking. Checking (fig. 8-9) consists of tiny breaks in only the upper coat or coats of the paint film without penetrating to the substrate. The pattern is usually similar to that of a crows foot. Cracking is larger with longer breaks extending through to the substrate (fig. 8-10). Both result from stresses exceeding the strength of the coating. But, whereas checking arises from stress within the paint film, cracking is caused by stresses between the film and the substrate.
Cracking generally takes place to a greater extent on wood, due to its grain, than on other substrates. The stress in the coating is greatest across the grain, causing cracks to form parallel to the grain of the wood. Checking and cracking are aggravated by excessively thick coatings that have reduced elasticity. Temperature variations, humidity, and rainfall are also concerns for checking or cracking.
|David L. Heiserman, Editor||
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Revised: June 06, 2015