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9/8/98 AC 43.13-1B SECTION 5. FINISHING WOOD STRUCTURES 1-64. GENERAL. Any repair to spars, ribs, skin surfaces, or other structural parts of the airframe involves finishing as the final step of the job. The surface finish is the final line of defense to prevent the destructive effects of moisture entry into the structure. The time and effort spent during the preparatory phase of the refinishing process will be reflected in the ap- pearance and longevity of the finished surface. Adherence to the instructions issued by the finish manufacturer is necessary to obtain the appearance desired and protective characteris- tics for the product used. The primary objec- tive of interior finishes is to afford protection of the wood against serious change in moisture content when exposed to damp air or to water that gains entrance to closed spaces by conden- sation or by penetration of rain, mist, or fog. Coatings, on contact areas between wood and metal protect the metal against corrosion from moisture in the wood. The primary objectives of the exterior finish are to protect the wood against weathering, provide a suitable appear- ance, and present a smooth surface in flight. 1-65. ACCEPTABLE FINISHES. Any varnish conforming to Federal Specification TT-V-109, as amended, or other coating ap- proved by the airframe manufacturer or the FAA is acceptable. Exterior surfaces must be further protected from the effects of abrasion, weather, and sunlight. A number of systems for exterior finishing have STC approval and are manufactured under a PMA. (See Chap- ter 2, Fabric Covering.) Low viscosity epoxy adhesive (meeting the requirements of para- graph 1-4 for that purpose) may be used as an internal surface finish when subsequent bond- ing is necessary. 1-66. PRECAUTIONS. a. When making repairs, avoid exces- sive contamination of surfaces with adhesive squeeze-out at joints and on all surfaces. Ex- cess adhesive should always be removed be- fore applying finish. Because many paints and adhesives are incompatible, even a slight amount of adhesive underneath the finish may cause premature deterioration of the finish. b. Soiling substances, such as oil and grease, should be removed as completely as possible. Naphtha may be used to remove sur- face deposits of oil and grease; however, thinned residue may penetrate into any unpro- tected wood. In areas where minor amounts of oil or grease have penetrated the wood surface, removal may be accomplished by use of an ab- sorbent type of cleaner such as gunsmith's whiting or a clothing spot lifter such as K2rTM. Marks that are made by grease pencils or lum- ber crayons containing wax are harmful and should be removed, but marks made by ordi- nary soft graphite pencils and nonoily stamp pad inks may be safely finished over. All dust, dirt, and other solid particles should be removed. c. Sawdust, shavings, and chips must be removed from enclosed spaces before they are sealed off by replacement of skin. A vacuum cleaner is useful for such cleaning. d. Since most adhesives will not bond satisfactorily to sealers, it is necessary to avoid applying sealer over the areas where adhesive will be applied. Mark off areas to receive ad- hesive, and allow an additional 1/4 inch on each side of the adhesive area to provide for Par 1-64 Page 1-35PDF Image | AFS-640
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