Many fiberglass and Kevlar$^{®}$ parts have a fine aluminum  mesh for lightning protection. This aluminum mesh often  corrodes around the bolt or screw holes. The corrosion affects  the electrical bonding of the panel, and the aluminum mesh  needs to be removed and new mesh installed to restore the  electrical bonding of the panel.  [Figure 7-23] Ultraviolet (UV) light affects the strength of composite  materials. Composite structures need to be protected by a  top coating to prevent the effects of UV light. Special UV  primers and paints have been developed to protect composite  materials. Erosion capabilities of composite materials have been  known to be less than that of aluminum and, as a result,  their application in leading-edge surfaces has been generally  avoided. However, composites have been used in areas of  highly complex geometry, but generally with an erosion  coating. The durability and maintainability of some erosion  coatings are less than ideal. Another problem, not as obvious  as the first, is that edges of doors or panels can erode if they  are exposed to the air stream. This erosion can be attributed  to improper design or installation/fit-up. On the other hand,  metal structures in contact or in the vicinity of these composite  parts may show corrosion damage due to inappropriate choice  of aluminum alloy, damaged corrosion sealant of metal parts  during assembly or at splices, or insufficient sealant and/or  lack of glass fabric isolation plies at the interfaces of spars,  ribs, and fittings.  [Figure 7-22]  A visual inspection is the primary inspection method for in- service inspections. Most types of damage scorch, stain, dent,  penetrate, abrade, or chip the composite surface, making the  damage visible. Once damage is detected, the affected area  needs to be inspected closer using flashlights, magnifying  glasses, mirrors, and borescopes. These tools are used to  magnify defects that otherwise might not be seen easily  and to allow visual inspection of areas that are not readily  accessible. Resin starvation, resin richness, wrinkles, ply  bridging, discoloration (due to overheating, lightning strike,  etc.), impact damage by any cause, foreign matter, blisters,  and disbonding are some of the discrepancies that can be  detected with a visual inspection. Visual inspection cannot  find internal flaws in the composite, such as delaminations,  disbonds, and matrix crazing. More sophisticated NDI  techniques are needed to detect these types of defects. $^{ }$Nondestructive Inspection (NDI) of  Composites Visual Inspection Corrosion Figure 7-22.  Erosion damage to wingtip. Figure 7-21.  Damage to radome honeycomb sandwich structure. 7-15