Reconditioning
Hammer Recontouring
In both grand and vertical pianos, the felt hammers become grooved, sometimes quite deeply, where they have been striking the strings. In a piano that is rarely used this may take decades to occur. In pianos that are heavily used, like practice room pianos in music schools which may get played many hours a day, it may only take a few years.
When hammers are new, only a very small area of the face of the hammers is striking the strings. When grooves are worn into the hammers, a half inch or more of the face of the hammers can be striking the strings, causing harsh and uneven tone.
You can see the grooves in an upright piano if you lift the lid and look straight down at the hammers from above. You will see something similar to the photo below.
An experienced technician can recontour the hammers, restoring their original shape. This not only prolongs the life of the hammers, but often improves the tone of the piano. When hammers are recontoured or "filed" correctly and expertly, the felt is taken off in layers until no string grooves remain and the original curve of the outline of the hammer is restored.
The hammers should not just look "clean". In an attempt to make the piano look like it has had work done on it, a Dremel tool is sometimes used just to quickly take the dirt off the hammers and make the felt look clean. This is simply cosmetic, and will not correct the problem. Upon closer inspection, grooves can still be easily detected on the hammers. This is an indication that they were not correctly recontoured.
A proper hammer recontouring job, as below, leaves the hammers with felt evenly removed all around the curve of the hammers. The striking surface must also be filed evenly "left to right" so it meets the strings evenly.
At some point in the piano's life, there may not be enough felt left on the hammers to allow recontouring, and the hammers must be replaced.