Sealant applied to aircraft exterior surfaces

However, the following restrictions apply to all sealant applications: • Sealant should be used within the application time limits specified by the sealant manufacturer. • Sealant should not be applied to metal that is colder than 70°F. Better adhesion is obtained and the applied sealant will have less tendency to flow while curing if the metal is warmed to a temperature between 90°F and 100°F before the sealant is applied. • Sealant should be discarded immediately when it becomes too stiff to apply or work. Stiff or partially cured sealant will not wet the surface to which it is to be applied as well as fresh material and, consequently, will not have satisfactory adhesion. • Sealant should not be used for faying surface applications unless it has just been removed from refrigerated storage or freshly mixed. While the use of sealants on aircraft surfaces has greatly increased over the past few years, application methods have been mostly through the use of brushes, dipping, injection guns, and spatulas. The spraying of sealants is a recent development. MIL-S- 81733 sealant, type III, is extensively used for spray application. If type III sealant cannot be procured, MIL-S-8802 sealant, class A, may be used by thinning it to a sprayable consistency by the addition of an appropriate solvent. Figure 14-44 shows sealant applied to an aircraft to protect some of the most corrosion-prone areas. The sealant was applied using spray, spatula, and brush methods. When you are pressure sealing an aircraft, the sealing materials should be applied to produce a continuous bead, film, or fillet over the sealed area. Air bubbles, voids, metal chips, or oily contamination will prevent an effective seal. Therefore, the success of the sealing operation depends upon the cleanliness of the area and the careful application of the sealant materials. There are various methods of pressure sealing the joints and seams in aircraft. The applicable structural repair manual will specify the method to be used in each application. The sealing of a faying surface is accomplished by brush coating the contacting surfaces with the specified sealant. The sealant should be applied immediately before fastening the parts together. 217.204 Figure 14-44.—Sealant applied to aircraft exterior surfaces. 14-46