Etching for 15 seconds on permanent tooth enamel provides adequate microporosity for resin adhesion and sealing and bond strengths great enough to result in cohesive fracture of the enamel when separated. On primary teeth, most research shows a 60-second etch is optimal, but some studies show 120 seconds provides more consistent bond strengths.
____
Ref.: Tooth-Colored Restoratives: Principles and Techniques
Link: http://www.amazon.com/Tooth-Colored-Restoratives-Principles-Harry-Albers/dp/1550091557/ref=sr_1_4?ie=UTF8&qid=1307640692&sr=8-4
