A 33 year old female patient complains of decayed tooth. IOPA reveals dentinal caries with 46 not involving the pulp. A composite restoration has been planned for the tooth. Following etching, how much time should the operator wait before the patient is allowed to rinse?: (A) 10 seconds, (B) 15 seconds, (C) 20 seconds, (D) 30 seconds
Answer is C. Once the tooth is etched, the acid should be rinsed away thoroughly with a stream of water for about 20 seconds, and then rinsed water must be removed.
When enamel alone is etched and is to be bonded with a hydrophobic resin (e.g. bisphenol A glycidyl methacrylate –based resin), it must be dried completely with warm air until it takes on a white, frosted appearance.
Dentin, in contrast, cannot withstand such aggressive drying, which would cause bond failure because of the formation of impermeable, collapsed collagen fibers.
In the total-etch technique, a dentin bonding agent and primer must be used that are compatible with both moist dentin and moist enamel.

Reference-  Sturdevant 6th ed Pg-310