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 (μTBS) 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 [σ_θ and σ_0.10 ] parameters) using the maximum likelihood method. The σ_θ and σ_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 μTBS values at σ_θ of failure but caused to decrease μTBS values at σ_0.10 of failure. DL application of SBU reduced μTBS values at σ_θ of failure. DL or EHR coating did not improve μTBS of SBU. EHR application increased μTBS of CSBU, whereas DL application did not benefit.