Do universal adhesives provide the benefits from double-application or an extra bonding layer in composite repair?

This study aimed to determine whether the application of extra hydrophobic resin (EHR) or double layer (DL) improves microtensile bond strength (mu TBS) of two universal adhesives to composite resin. Composite blocks were fabricated and exposed to thermal cycles. The specimens were horizontally sectioned into two halves. Scotch Bond Universal (SBU) and Clearfil S3 Bond Universal (CSBU) were applied to the ground composite surface according to the manufacturers' instructions, or with DL application or EHR application. The repair composite was incrementally placed to bonded planes. Composite sticks were achieved and tensed with a universal testing machine, followed by examining the fracture surfaces by a scanning electron microscope. Data were evaluated by Weibull analysis (shape and scale [sigma(theta) and sigma(0.10)] parameters) using the maximum likelihood method. The sigma(theta) and sigma(0.10), respectively, estimate strength at 63.2 and 10% probability of failure. Shape parameter values showed significant differences among treatments. DL application of CSBU did not affect mu TBS values at sigma(theta) of failure but caused to decrease mu TBS values at sigma(0.10) of failure. DL application of SBU reduced mu TBS values at sigma(theta) of failure. DL or EHR coating did not improve mu TBS of SBU. EHR application increased mu TBS of CSBU, whereas DL application did not benefit.