CoatingsPro Magazine

MAY 2016

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Page 34 of 84

34 MAY 2016 COATINGSPROMAG.COM above types of imperfections at that time, prior to the application of an additional coat. Te coating should be repaired for damage as well as for imper- fections prior to the following coat. Each coat should be checked for curing and drying conditions; the inspector should determine that the drying time is proper both between coats and for the fnal coat. He or she should make certain that there has been no condensation on the surface or fallout of contamination between coats. If such conditions have occurred, each coat should be cleaned prior to additional coats. Finished surfaces should be inspected. Tis inspection should cover overspray, pinholes, runs, holidays, and any area that appears to be rough or improperly applied. All such imperfections should be marked and repaired prior to accep- tance of the coating. The Importance of Temperature During the application, the inspector should be aware of temperatures; not only the ambient temperature of the atmosphere, but also the temperature of the metal. Te relative humidity should be checked to ensure that condensation and moisture are not possible on the surface during the application. W hen the temperature is close to the dew point, even the evaporation of the solvent in the coating can reduce the surface tempera- ture condition to the point where moisture can condense on the surface. W hen this happens, the coating may blush (which is the absorption of the condensed moisture into the coating), causing a poor flm. Te minimum air temperature usually permitted for the application of coatings is 40° F (4.4° C), and the temperature of the steel should never be much lower than that. Under certain conditions, and with certain coatings, the temperature limits can be lowered, but it is generally not good coating practice to coat when the metal temperature is below 35° F (1.7° C). Coating during below-freezing temperatures, although possible with some coatings, is always dangerous since any moisture that may have been on the surface turns to either frost or ice and will cause immediate delamination of the coating. High temperatures are also a problem. Generally, coatings should not be applied when metal temperatures are greater than 125° F (51.7° C). Some special coatings may be applied at higher temperatures, although these are usually the exception. Te temperature at which it is uncomfortable to place a hand on the surface, and usually not possible to hold it for any period of time, is 125° F (51.7° C). Te temperature of the surface and the air are always a concern during an application. Te contractor may want to continue coating even though the minimum or maximum temperature has been exceeded. At this point, the inspec- tor must determine whether it is possible to exceed the limits or whether, because of conditions, the quality of the coating will sufer. Generally, it is best not to permit any broadening of the tempera- ture range since coating problems only multiply at each end of the range. If coating is done on the exterior surfaces, weather changes are always a problem, and may be harmful to the freshly applied coating. Rain, sharp decreases in temperature, increases in wind conditions, and increases in humidity should be carefully watched. If possible, the application should be halted prior to any damage to the coating or, at least, time should be provided for the applied coating to dry before damage can occur due to such changes. Awareness of Humidity As previously discussed, condensation is a particularly difcult factor to not only observe, but also control. A surface can change within a matter of minutes from being dry to being so wet that you could write your name in the conden- sation. Tis is the reason that humidity should be checked periodically, making certain that the dew point is sufciently removed from the ambient tempera- ture so that condensation cannot take place. Any time that the steel temperature is lower than the ambient outside temperature, condensation problems are possible. Tis is particu- larly true where there are heav y steel masses, such as heav y plate or heav y cross sections of steel. Te tempera- ture of the steel, after being exposed overnight, takes a considerable period of time to increase to the ambient exterior temperature. In fact, in many cases it does not rise to the air tempera- ture even over an entire day's period. Tis makes the surface thermometer an extremely handy tool for the inspec- tor. Each day's work should be fnished in considerable advance of the time when it is possible for condensation to occur on the surface in order to allow a reasonable drying period. CP Tis article is based on an excerpt from Chapter 18 of the third edition of Corrosion Prevention by Protective Coatings, which was released in Spring 2014. Originally written by Charles G. Munger, the second and third editions of the book were edited and updated by Associate Author Louis D. Vincent, Ph.D. Te previous edition of the book was published in 1999. For more information, contact: Finished surfaces should be inspected. This inspection should cover overspray, pinholes, runs, holidays, and any area that appears to be rough or improperly applied. Inspector's Corner

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