## 1 Introduction  
The methods used to apply adhesives can considerably influence the clinical outcomes, regardless of the adhesive system utilized [1, 2]. Previous studies investigated the bond effectiveness of various adhesive systems using different adhesive application methods and concluded that bond performance and morphological features in the vicinity of the resin-tooth interface were strongly influenced by the adhesive application method [3, 4, 5, 6]. For instance, a warm air blow technique [1], the etching mode [4], an active application technique [5, 6], and double-layer application [7, 8, 9] have been reported to improve the bonding performance of universal adhesives. Some of these techniques also influence the morphological features at the interface between the adhesive layer and the tooth substrate and the thickness of the adhesive layer [7, 8, 9]. The configuration of the tooth cavity is highly variable--depending on the location, depth, and size of the lesion, and the defects of the tooth substrate--and can influence not only the contraction stress of the resin composite but also the methods of adhesive application, air blowing, and filling [10, 11]. The thickness of the adhesive layer might vary within the cavity. Furthermore, the thickness of the adhesive layer depends on the viscosity of the adhesive, the method used to apply the adhesive, and the strength of the air blow stream [7; 8; 9]. When comparing the thickness of the adhesive layer between multiple-step adhesive systems and single-step self-etch adhesive systems, including universal adhesive systems, the adhesive layers in the three-step etch-and-rinse and two-step self-etch adhesive systems were approximately five times thicker than those in the single-step self-etch adhesives [12]. A thicker adhesive layer is thought to improve the bond performance because it reduces the concentration of local stress, suppresses crack propagation, and decreases the relative thickness of the oxygen inhibited layer [7; 8; 9]. [PERSON] et al. [13] investigated the effect of consecutive coatings of the three-step adhesive system on the dentin microtensile bond strength and concluded that the bond strengths increased with each coating, up to four coats; no increase in bond strength was observed after more than four coats. However, the optimal thickness of the adhesive layer remains controversial and may vary among the different adhesive systems and tooth substrates. Phosphoric acid pre-etching has profound effects on the immediate and long-term bond durability because of the creation of a strong mechanical interlocking between the adhesive layer and the enamel substrate [14]. A recently introduced two-step adhesive system comprises a 2-hydroxyethyl methacrylate (HEMA)-free universal adhesive-derived primer and a hydrophobic bonding agent without any solvents or functional monomers. This adhesive system showed superior bond durability under thermal stress and long-term water storage when compared with conventional two-step self-etch adhesives, regardless of the tooth substrate or etching mode used [15]. Moreover, [PERSON] et al. [16] showed that the enamel and dentin bond durability achieved under fatigue stress by the new two-step adhesive system was equivalent to that attained by a gold standard two-step self-etch adhesive, Clearfil SE Bond. Thus, this new adhesive system might be able to achieve a durable bond performance and can be used in the clinical setting owing to the presence of a universal adhesive-derived primer. However, there is little information regarding the bond performance of this two-step adhesive system when applied using different methods. This study aimed to investigate the effect of the adhesive application method on the durability of the enamel bond and the thickness of the adhesive layer using the new two-step adhesive system and compare it with those of the conventional two-step self-etch adhesive systems.