Interior cleaning is technically a partial interior reconditioning (see also under reconditioning).
This is a very important issue in the U.S. North-East because the very high humidity in the summer causes damage to the interior of the piano.
The moisture condensates on all the unfinished metal parts inside the piano (as those are colder than the air temperature) that causes the strings and the tuning pins to develop an oxid-layer and become rusty over the years.
As a result – besides becoming visually unattractive – the piano will lose its sound clarity and high partials, as it becomes sounding “nasal”.
Unfortunately most of the technicians who clean the interior of pianos – while removing the rust – also scratch the strings, because they don’t use the correct grade abrasives as they are not concerned about the sound of the piano – causing an overall greater damage!
Interior cleaning is a very delicate job, therefore it should be done only by a professional!