Step 6: Finishing After the patch has cured, a conducting mesh and finish coat  should be applied if needed. Trailing edges of control panels are highly vulnerable to  damage. The aft 4 inches are especially subject to ground  collision and handling, as well as to lightning strike. Repairs  in this region can be difficult because both the skins and the  trailing edge reinforcement may be involved. The repairs to  a honeycomb core on a damaged edge or panel are similar to  the repair of a sandwich honeycomb structure discussed in  the Damage Requiring Core Replacement and Repair to One  or Both Faceplate Repair sections. Investigate the damage,  remove damaged plies and core, dry the part, install new core,  layup the repair plies, curing and post inspection. A typical  trail edge repair is shown in  Figure 7-66. Resin injection repairs are used on lightly loaded structures  for small damages to a solid laminate due to delamination.  Two holes are drilled on the outside of the delamination area  and a low-viscosity resin is injected in one hole until it flows  out the other hole. Resin injection repairs are sometimes  used on sandwich honeycomb structure to repair a facesheet  disbond. Disadvantages of the resin injection method are that  the fibers are cut as a result of drilling holes, it is difficult to  remove moisture from the damaged area, and it is difficult to  achieve complete infusion of resin.  [Figure 7-67] Composite materials can be used to structurally repair, restore,  or enhance aluminu m, steel, and titanium components.  Bonded composit doublers have the ability to slow or  stop fatigue crack growth, replace lost structural area due  to corrosion grindouts, and structurally enhance areas with  small and negative margins. Boron epoxy, GLARE $^{®}$, and graphite epoxy materials have  been used as composite patches to restore damaged metallic  wing skins, fuselage sections, floor beams, and bulkheads.  As a crack growth in hibitor, the stiff bonded composite  materials constrain the cracked area, reduce the gross stress  in the metal, and provide an alternate load path around the  crack. As a structural enhancement or blendout filler, the  high modulus fiber composites offer negligible aerodynamic  resistance and tailorable properties. Surface preparation is very important to achieve the adhesive  strength. Grit blast silane and phosphoric acid anodizing are  used to prepare aluminum skin. Film adhesives using a 250 °F  (121 °C) cure are used routinely to bond the doublers to the  metallic structure. Critical areas of the installation process  include a good thermal cure control, having and maintaining  water-free bond surfaces, and chemically and physically  prepared bond surfaces.  Secondarily bonded precured doublers and in-situ cured  doublers have been used on a variety of structural geometries  ranging from fuselage frames to door cutouts to blade  stiffeners. Vacuum bags are used to apply the bonding and  curing pressure between the doubler and metallic surface.  Fiberglass molded mats consists of short fibers, and the  strength is much less than other composite products that  use continuous fibers. Fiberglass molded mats are not used  for structural repair applications, but could be used for non- Fiberglass Molded Mat Repairs Figure 7-66.  Trailing edge repair. Figure 7-67. Resin injection repair. Composite Patch Bonded to Aluminum Structure Trailing Edge and Transition Area Patch Repairs Resin Injection Repairs 20 psi air Drill holes Inject resin Delamination Skin Injection gun Nonstructural sanding ply (adhesive film or fiberglass prepreg) Extra repair ply Third repair ply Second repair ply First repair ply Adhesive film Masking tape (3.0 to 4.0 wide) Taper sand 7-40