Figure 8-7.—Wrinkling.ChalkingChalking (fig. 8-8) is the result of paintweathering at the surface of the coating. The vehicle isbroken down by sunlight and other destructive forces,leaving behind loose, powdery pigment that can easily berubbed off with the finger. Chalking takes place rapidlywith soft paints, such as those based on linseed oil.Chalking is most rapid in areas exposed to sunshine. Inthe Northern Hemisphere, for example, chalking is mostrapid on the south side of a building. On the other hand,little chalking takes place in areas protected fromsunshine and rain, such as under eaves or overhangs.Controlled chalking can be an asset, especially in whitepaints where it acts as a self-cleaning process and helpsto keep the surface clean and white. The gradualwearing away reduces the thickness of the coating, thusallowing continuous repainting without making thecoating too thick for satisfactory service.Do not use a chalking or self-cleaning paintabove natural brick or other porous masonry surfaces.The chalking will wash down and stain or discolor theseareas.Chalked paints are generally easier to repaintsince the underlying paint is in good condition andrequires little surface preparation. But, this is not thecase with water-thinned paints; they adhere poorly tochalky surfaces.Checking and CrackingChecking and cracking are breaks in a coatingformed as the paint becomes hard and brittle.Temperature changes cause the substrate and overlyingpaint to expand and contract. As the paint becomeshard, it gradually loses its ability to expand withoutbreaking. Checking (fig. 8-9) consists of tiny breaks inonly the upper coat or coats of the paint filmFigure 8-9.—Severe checking.Figure 8-8.—Degrees of chalk.8-13
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