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AC 43.13-1B 9/8/98 surfaces. Sanding should never be continued to the extent that it alters the flatness of the surface. Very light sanding may also improve the wetting of the adhesive to very hard or resinous materials. b. Wetting tests are useful as a means of detecting the presence of wax, old adhesive, and finish. A drop of water placed on a sur- face that is difficult to wet and thus difficult to bond will not spread or wet the wood rapidly (in seconds or minutes). The surface may be difficult to wet due to the presence of wax, ex- posure of the surface to heat and pressure as in the manufacture of hot press bonded plywood, the presence of synthetic resins or wood ex- tractives, or simply chemical or physical changes in the wood surface with time. Good wettability is only an indication that a surface can be bonded satisfactorily. After performing wetting tests, allow adequate time for wood to dry before bonding. Preliminary bonding tests and tests for bond strength are the only posi- tive means of actually determining the bonding characteristics of the adhesive and material combinations. (See paragraph 1-29h.) 1-7. APPLYING THE ADHESIVE. To make a satisfactory bonded joint, spread the adhesive in a thin, even layer on both surfaces to be joined. It is recommended that a clean brush be used and care taken to see that all sur- faces are covered. Spreading of adhesive on only one of the two surfaces is not recom- mended. Be sure to read and follow the adhe- sive manufacturer’s application instructions. 1-8. ASSEMBLY TIME IN BONDING. Resorcinol, epoxy, and other adhesives cure as a result of a chemical reaction. Time is an im- portant consideration in the bonding process. Specific time constraints are as follows: a. Pot life is the usable life of the adhesive from the time that it is mixed until it must be spread onto the wood surface. Once pot life has expired, the remaining adhesive must be discarded. Do not add thinning agents to the adhesive to extend the life of the batch. b. Open assembly time is the period from the moment the adhesive is spread until the parts are clamped together. Where surfaces are coated and exposed freely to the air, some adhesives experience a much more rapid change in consistency than when the parts are laid together as soon as the spreading has been completed. c. Closed assembly time is the period from the moment that the structure parts are placed together until clamping pressure is ap- plied. The consistency of the adhesive does not change as rapidly when the parts are laid together. d. Pressing (or clamping) time is the pe- riod during which the parts are pressed tightly together and the adhesive cures. The pressing time must be sufficient to ensure that joint strength is adequate before handling or ma- chining the bonded structure. NOTE: Follow the adhesive manufac- turer’s instructions for all time limits in the bonding process. If the recom- mended open or closed assembly peri- ods are exceeded, the bond process should not be continued. Discard the parts if feasible. If the parts cannot be discarded, remove the partially cured adhesive and clean the bond line per adhesive manufacturer’s instructions before application of new adhesive. 1-9. BONDING TEMPERATURE. Tem- perature of the bond line affects the cure rate of the adhesive. Some adhesive types, such as resorcinol, require a minimum temperature which must be maintained throughout the Page 1-6 Par 1-6PDF Image | AFS-640
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