in areas where pressurized cavities must be maintained. Drying sealants set and cure by evaporation of the solvent. The solvents in these sealants provide the desired consistency for application. Consistency or hardness may change when this type of sealant dries, depending upon the amount of solvent it contains. Shrinkage is a consideration when these sealants are used. Shrinkage occurs upon drying. The degree of shrinkage depends on the proportion of solvents. Catalyst-cured sealants have advantages over drying sealants. They are transformed from a fluid or semifluid state into a solid mass by chemical reaction of physical change rather than by evaporation of a solvent. A chemical catalyst of accelerator is added and thoroughly mixed just before sealant applications. Heat may or may not be used to speed up the curing process.    When a catalyst is used, accurate proportioning and thorough mixing of the two components are very important to assure a complete and even cure. Application of Sealants Application of sealants varies according to time element, tools required, and the method of application. However, the following restrictions apply to all sealant applications: Sealants should be used within the approximate application time limits specified by the sealant manufacturer. Sealants should not be applied to metal that is colder than 70°F. Better bonding is obtained and the applied sealant will have less tendency to flow out of place while curing if the metal is warmed to a temperature of 90°F to 100°F before the sealant is applied. Sealants should be discarded immediately when they become too stiff to apply or work readily. Stiff or partially cured sealants do not wet the surface to which they are applied as well as fresh material. This causes uneven bonding. Sealants should not be used for close-fitting (faying) surface applications unless they have just been removed from refrigerated storage or freshly mixed. Brushes, dipping, injection guns, spatulas, and spray guns are the methods used to apply sealants. Figure 4-35 shows (black areas) where sealant is applied to protect some of the most corrosion-prone areas on an F-14 aircraft. The sealant is applied by using the spray, spatula, and brush methods. Sealant MIL-S-81733, type III, is the sealant used most extensively for spray application. If type III sealant cannot be procured, sealant MIL-S-8802, class A, may be used by thinning it to a sprayable consistency by the addition of the correct solvent. When an aircraft is pressure sealed, the sealing materials should be applied as a continuous bead, film, or fillet over the sealed area. Air bubbles, voids, metal chips, or oily contamination 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 joints and seams in aircraft. The applicable SRM will specify the method to be used in each application. The sealing of a faying surface is done by brush. The contacting surfaces are coated with the specified sealant. Application of the sealant should be made immediately before the parts are fastened together. Careful planning of work and equipment are necessary so faying surface seals on large assemblies may be closed within the application time limit of the sealant. Once the sealant has been applied, the parts must be joined, the bolts torqued, and the rivets driven all within the application time limit. When insulating tape has been installed between the faying surfaces to prevent contact of dissimilar metals, pressure sealing should be done by fillet sealing. In fillet sealing, the sealant is spread along the seam with a sealant injection gun in about 3-foot increments. Before proceeding to the next increment, the applied portion of the fillet is worked in with a sealant spatula or tool (fig. 4-36). This working of the sealant is done to till in all voids in the seam and to eliminate most air bubbles. The care used in working out the air bubbles determines the leakfree service life of the sealant. After the sealant has cured to a tackfree condition, the fillet should be inspected for remaining air bubbles. These air bubbles should be opened and filled with sealant. When a heavy fillet is required, the fillet should be applied in layers. The top layer should fair with the metal. Injection sealing is the pressure filling of openings or voids with a sealant injection gun. The sealant is forced into the opening until it emerges from the opposite side. Voids and cavities are filled by starting with the nozzle of the sealant injection gun at the bottom of the space and tilling as the nozzle is 4-51