|
|
Selant Repair All surfaces that are to be sealed must be thoroughly cleaned to ensure maximum adhesion between the surface and the sealant. Only approved strippers, cleaners, and sealants can be used; and, even though approved, the stripper compound must be used with care. Details on cleaning and the sealant to be used can be found in the technical manual applicable to the aircraft. The paragraphs that follow give general information on cleaning techniques, safety practices, application, and curing. Cleaning Techniques. Loose foreign material can be removed by using a vacuum cleaner on the area to be resealed. The old sealant can be scraped from the repair area with a sharp plastic, phenolic, or hardwood block. Care must be taken not to make any scratches. A stripper and cleaner are used next to remove all the old sealant. Cleaners in general use are aliphatic naphtha, ethyl acetate, or lacquer thinner. The naphtha can be used satisfactorily by dipping a clean cloth or brush into the naphtha and then scrubbing the surface. After using aliphatic naphtha, a clean cloth must be used to wipe metal surfaces dry. The cleaner must not be allowed to dry on a metal surface. The surface can be checked for cleanness by pouring clean water over it, after it has been wiped dry of the cleaner. If the surface is not free of an oily film, the water will separate into small droplets. Safety Practices. In addition to protecting undamaged sealants and acrylic plastic from the stripper compound, users must practice personal safety. For example, if artificial lighting is used when a repair is being made, the light must be explosion proof. Clothing that prevents these chemicals from touching the skin and goggles to protect the eyes must be worn. Ventilation to dissipate fumes must be ensured, and no smoking regulations must be enforced. Sealant Application. Rubber seals must be applied, or installed, immediately after the seal frame is cleaned. A clean paint brush can be used to apply an even coat of rubber cement upon the metal parts and the seal surfaces to be joined. If the cement needs thinning, apliphatic naphtha can be used. The cement must be allowed to dry until it becomes quite sticky before joining the seal to the metal. The seal is joined to the metal by pressing it firmly along all contact points. Next, the seal retainers are installed, and the seal is allowed to set for 24 hours before using. Toluene, or an approved solvent, can be used to clean the brushes used to apply the cement. Sealing compounds are applied only when the contacting surfaces are clean. The compound must be spread from the tube by using a continuous forward movement to the pressure side of the joint. The compound is spread 3 inches beyond each end of the repair area. If the compound is in bulk form it is applied with a pressure gun. Curing. Two layers or coats of compound are frequently required. If two coats are needed, the first coat must be allowed to cure before the second is applied. The compound must cure until it becomes tough and rubbery before the surfaces are joined. Curing time varies with temperature. High temperatures shorten and low temperatures lengthen curing time. Artificial heat can be used to speed curing, but care must be used to avoid damaging the sealant with too high a temperature. Warm circulating air, not over 120° F (49° C), or infrared lamps placed 18 inches or more from the sealants are satisfactory heat sources. If infrared lamps are used, adequate ventilation must be available to carry away solvent fumes.
|
