The strings are normally replaced during a complete rebuilding. The tenor or middle and treble (right-hand side) piano strings seldom go bad. The main reason to replace these is if they are rusting or breaking or if the tuning pins are slipping. If they are only slightly rusted they can be cleaned and polished. Replacing the copper-wrapped bass strings is often done when they sound dull and “tubby” or hiss. New strings are always a part of a rebuilding.