The phrase "inside surface" refers to the interior or inner side of an object, such as the inside of a container or the side facing inward. Full definition
Also spray the entire inside surface of dog houses and kennels. (pet-lock.com)
Brush your child's teeth gently on both the outside and inside surfaces twice a day. (babycenter.com)
Spoon 1 teaspoon of olive oil from the dipping sauce (just the oil) into the baking sheet and spread the oil on the entire inside surface. (lowcarbmaven.com)