# Effectiveness of Bonding Steel Elements with Polyester-Coated Paint
## 4 Discussion
Moreover, based on the ASTM D5946 standard, it can also be assumed that the joined materials (in three variants of the samples) show high wetting properties because the wetting angle is less than 71\({}^{\circ}\). However, it should be noted that, according to the referenced standard, the contact angle itself is not a completely acceptable measure of paint, coating, or adhesive adhesion, but the contact angle values can be used to determine the level of surface treatment. It should also be emphasized that, in many works (e.g., see [PERSON] [2], [PERSON] et al. [34], [PERSON] and [PERSON] [37], [PERSON] and [PERSON] [48]), the contact angle value has been used to determine the wettability of a surface. The authors of [21] presented the influence of the surface treatment on the adhesive properties of the steel sheet surfaces and the strength of adhesive joints, in which paint coatings were also analyzed. It has been shown that the surfaces of steel samples show a similar contact angle value (60.47\({}^{\circ}\)), and slightly different contact angle values were obtained after various surface treatments with paint coatings, which means that special attention should be paid to the development of technology for preparing the surface of elements for adhesion processes. On the basis of the information obtained, it was noted that the presence of a paint coating increases strength, although it depends on the type of adhesive used. It is possible that the coating may increase the strength of the adhesive bond, depending on the materials of the coating, the method of its application, and the type of substrate material to which it is applied. Furthermore, it is likely that a different difference can be observed between the adhesive strength of uncoated and coated materials. Coatings can perform various functions and enhance joint performance in various ways. In [5], the effect of cataphoretic and powder coatings and also the method used to apply the primer on the adherend's surface on the strength and the types of failure of the aluminum alloy adhesive joints was presented. Based on the test results, it was found that both the method of applying the adhesion promoter and the presence of the cataphoretic coating had an impact on the strength of the adhesive joints. The use of an adhesion promoter significantly affects the strength of both uncoated aluminum alloy adhesive joints and powder-coated joints. In this article, the adhesion promoter was applied before applying the paint coat, and for this case, we obtained the highest strength values of the adhesive joints of the steel samples with paint coatings with a previously applied primer in individual groups of joints made with the epoxy and polyurethane adhesives. Therefore, it can be assumed that the presence of a primer in the system of joined materials is conducive to obtaining strong adhesive joints. The surface treatment of the sheet metal surface before applying paint coatings may also affect the final strength of adhesive joints, and it is worth paying attention to this consideration when applying paint coatings. The research presented by [PERSON] et al. [41] and also that of the authors of [49] propose that both the type of coating and the type of adherend have a great impact on adhesion. [PERSON] et al. [19] also pointed out that the type of coating, as well as the type of substrate, affects the strength of the adhesive joints. Taking into account the aspect of preparing the surface of steel sheets before bonding, attention should be paid to the technology used to apply the paint coatings (with and/or without a primer), the availability of devices and whether their use is time-consuming, their protective and functional properties, the strength required for the adhesive joints, and the value of the transferred loads. If strength is required at the level of the assembly joints and the tightness of the joints is highly important, a specific type of adhesive can be selected.