Enamel should be etched prior to use of any universal adhesive for several reasons. First, the pH of universal adhesives (around 3) is less acidic than previous self-etch primers (around 2) and phosphoric acid (less than 1). Second, universal adhesives do not provide a good etch pattern on enamel when observed under scanning electron microscopy. Finally, we performed a clinical trial with a common universal adhesive and reported that there was less marginal discoloration and less restoration debonding when enamel was etched.
When bonding to dentin, a self-etching technique can be used; however, it is important to agitate the adhesive, adequately evaporate all solvent from the adhesive, and adequately cure the adhesive.  The optimal bonding technique is the selective etch method, in which enamel is etched with phosphoric acid and dentin is self-etched using a universal adhesive.