Is it necessary to clean the outer surface of the sock from anything that might prevent the water of masḥ (wiping) from reaching it, or is it limited to removing whatever prevents water from reaching the skin in the case of ghusl (washing)?

Whatever prevents water from reaching the sock during wiping, such as attached hair or wool, must be removed, as it is considered a barrier. Scholars disliked repeating the wiping on the khuff or tracing its parts, because wiping is based on ease and concession.