•  Open assembly time—period of time between the  application of the adhesive and the assembly of the  joint components. •  Closed assembly time—time elapsing between the  assembly of the joints and the application of pressure. •  Pressing or clamping time—time during which  the components are pressed tightly together under  recommended pressure until the adhesive cures (may  vary from 10 to 150 pounds per square inch (psi) for  softwoods, depending on the viscosity of the glue). •  Caul—a clamping device, usually two rigid wooden  bars, to keep an assembly of flat panel boards aligned  during glue-up. It is assembled with long bolts and  placed on either side of the boards, one on top and  another below, and parallel with the pipe/bar clamps.  A caul is usually finished and waxed before each use  to keep glue from adhering to it.  •  Adhesive pot life—time elapsed from the mixing  of the adhesive components until the mixture must  be discarded, because it no longer performs to its  specifications. The manufacturer’s product data sheet  may define this as working time or useful life; once  expired, the adhesive must not be used. It lists the  specific temperature and quantity at which the sample  amount can be worked. Pot life is a product of time  and temperature. The cooler the mix is kept, within  the recommended temperature range, the longer it  is usable.  1.  Proper and equal moisture content of wood to be joined  (8 to 12 percent). 2.  Properly prepared wood surfaces that are machined  or planed, and not sanded or sawed. 3.  Selection of the proper adhesive for the intended task,  which is properly prepared and of good quality.  4.  The application of good gluing techniques, including  fitment, recommended assembly times, and adequate  equal pressure applied to the joint. 5. Performing the gluing operation under the  recommended temperature conditions.  The surfaces to be joined must be clean, dry, and free from  grease, oil, wax, paint, etc. Keep large prepared surfaces  covered with a plastic sheet or masking paper prior to the  bonding operation. It is advisable to clean all surfaces with  a vacuum cleaner just prior to adhesive application.  Smooth even surfaces produced on planers and joiners  with sharp knives and correct feed adjustments are the best  surfaces for gluing solid wood. The use of sawn surfaces  for gluing has been discouraged for aircraft component  assembly because of the difficulty in producing a surface  free of crushed fibers. Glue joints made on surfaces that are  covered with crushed fibers do not develop the normal full  strength of the wood.  Some of the surface changes in plywood, such as glazing  and bleed-through, that occur in manufacture and may  interfere with the adhesion of glue in secondary gluing  are easily recognized. A light sanding of the surface with  220-grit sandpaper in the direction of the grain restores the  surface fibers to their original condition, removes the gloss,  and improves the adhesion of the glue. In contrast to these  recognized surface conditions, wax deposits from cauls used  during hot pressing produce unfavorable gluing surfaces that  are not easily detected. Wetting tests are a useful means of detecting the presence of  wax. A finely sprayed mist or drops of water on the surface of  wax-coated plywood bead and do not wet the wood. This test  may also give an indication of the presence of other materials  or conditions that would degrade a glue joint. Only a proper  evaluation of the adhesion properties, using gluing tests,  determines the gluing characteristics of the plywood surfaces. Satisfactory glue joints in aircraft should develop the  full strength of the wood under all conditions of stress.  To produce this result, the conditions involved in the  gluing operation must be carefully controlled to obtain a  continuous, thin, uniform film of solid glue in the joint  with adequate adhesion to both surfaces of the wood. These conditions required: The manufacturer’s directions should be followed for the  preparation of any glue or adhesive. Unless otherwise specified  by the glue manufacturer, clear, cool water should be used  with glues that require mixing with water. The recommended  proportions of glue, catalyst, and water or other solvent should  be determined by the weight of each component. Mixing can  be either by hand or machine. Whatever method is used, the  glue should be thoroughly mixed and free of air bubbles,  foam, and lumps of insoluble material. To make a satisfactorily bonded joint, it is generally desirable  to apply adhesive to both surfaces and join in a thin even layer.  The adhesive can be applied with a brush, glue spreader, or  a grooved rubber roller. Follow the adhesive manufacturer’s  application instructions for satisfactory results.  Preparation of Wood for Gluing Preparing Glues for Use Applying the Glue/Adhesive  6-11