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1. Surface treatment. By treating the surface, conditions for good adhesion can be created, thereby increasing the bonding strength. 2. Cleaning and degreasing. For polyurethane adhesives, the oils on metal surfaces are incompatible with polyurethane, and the moisture present reacts with the NCO groups in the adhesive to form bubbles. This reduces the surface area of contact between the adhesive and the substrate, as well as the cohesion within the adhesive layer; therefore, surface cleaning is necessary before bonding. 3. Roughening treatment. Smooth surfaces generally need to be textured in order to increase the contact area between the adhesive and the substrate; by penetrating into the recesses on the surface of the substrate, the adhesive can bond the substrate together firmly. Common methods for this purpose include sandblasting. 4. Apply the top coating. To improve adhesion properties, a very thin layer of primer can be applied to the surface of the treated substrate; the primer also protects the surface of the material to be bonded from corrosion and contamination, thereby extending its storage time. 5. Gluing. Applying glue is the process of applying an adhesive to the surface of the object to be bonded in an appropriate manner. Different adhesives require different application methods, among which brushing is the most common. The glue-brushing method involves using a brush to apply the glue to the surface of the object to be bonded. For frames with a rough surface, it is necessary to apply the glue back and forth; for smooth surfaces, the glue can be applied in one direction, but at a slow pace to prevent bubbles from forming, ensuring an even distribution of the glue, especially in the central areas. The average thickness is 0.015–0.020 mm (single coating). Make it as thin as possible while ensuring there is no lack of adhesive. 6. Let it air dry. After applying the glue, allowing it to dry properly helps to remove air, even out the glue layer, and increase its viscosity. Solvent-based adhesives must be allowed to dry so that the solvents can evaporate; otherwise, the cured adhesive layer will have a loose structure with pores, resulting in reduced bonding strength. Each type of adhesive requires a different drying time; if it dries for too long, its stickiness is greatly reduced and it can no longer be used for bonding. 7. Curing of the adhesive. Most polyurethane adhesives do not possess high bonding strength immediately upon bonding; they require curing first. Ovens are used as heating methods for curing. The three important parameters of the curing process are temperature, pressure, and time. Among them, temperature has the greatest impact. The main factors affecting bonding are the adhesive, the material to be bonded, and the bonding process technology.