Be careful to ensure the surfaces make good contact and the  joint is positioned correctly before applying the adhesive.  Keep the open assembly time as short as possible and do  not exceed the recommended times indicated in the product  data sheet.  pressure to poorly machined or fitted wood joints usually  results in a thick glue line, indicating a weak joint, and should  be carefully avoided.  High clamping pressure is neither essential nor desirable,  provided good contact between the surfaces being joined is  obtained. When pressure is applied, a small quantity of glue  should be squeezed from the joint. This excess should be  removed before it sets. It is important that full pressure be maintained on the joint for the entire cure time of the adhesive  because the adhesive does not chemically relink and bond if  it is disturbed before it is fully cured. To ensure the maximum strength of the bonded surfaces,  apply even force to the joint. Non-uniform gluing pressure  commonly results in weak areas and strong areas in the  same joint. The results of applied pressure are illustrated in  Figure 6-11. The full curing time of the adhesive is dependent on the  ambient temperature; therefore, it is very important to  follow the manufacturer’s product data sheets for all phases  of the gluing operation from the shelf life to the moisture  content of the wood to the proper mixing of the adhesive  to the application, and especially to the temperature.  The successful assembly and fabrication depends on the  workmanship and quality of the joints and following the glue  manufacturer’s instructions. Use pressure to squeeze the glue out into a thin continuous  film between the wood layers, to force air from the joint, to  bring the wood surfaces into intimate contact with the glue,  and to hold them in this position during the setting of the glue.  Pressure may be applied by means of clamps, elastic straps,  weight, vacuum bags, or other mechanical devices. Other  methods used to apply pressure to joints in aircraft gluing  operations range from the use of brads, nails, and screws to  the use of electric and hydraulic power presses.  All gluing operations should be performed above 70 °F for  proper performance of the adhesive. Higher temperatures  shorten the assembly times, as does coating the pieces of  wood with glue and exposing openly to the air. This open  assembly promotes a more rapid thickening of the glue than  pieces being mated together as soon as the spreading of the  glue is completed.  The amount of pressure required to produce strong joints in  aircraft assembly operations may vary from 10 to 150 psi for  softwoods and as high as 200 psi for hardwoods. Insufficient  Figure 6-12  provides an example of resorcinol resin glue and  the allowable assembly times and gluing pressure when in  the open and closed assembly condition. All examples are  for an ambient temperature of 75 °F. Figure 6-13  provides examples of strong and weak glue joints  resulting from different gluing conditions. A is a well-glued  joint with a high percentage of wood failure made under  proper conditions; B is a glue-starved joint resulting from  the application of excessive pressure with thin glues; C is a  dried glue joint resulting from an excessively long assembly  time and/or insufficient pressure . Figure 6-11. Even distribution of gluing pressure creates a strong,  gap-free joint. Figure 6-12.  Examples of differences for open and closed assembly times. Arrows indicate pressure Pressure block Pressure block Gap Glue Gluing Pressure Closed Open Closed Open Type of Assembly Up to 50 minutes Up to 12 minutes Up to 40 minutes Up to 10 minutes Maximum Assembly Time 100–250 psi 100–250 psi Less than 100 psi Less than 100 psi Resorcinol resins Pressure on the Joint 6-12