When a hammer hit strings of a note, for the max energy transfer, all strings of this note must be struck by the hammer top surface at the same time. Not all strings are at the same level/elevation/height. The hammer tops will need to be filed to achieve this.
sLift the dampers by pressing sustain pedal, lift each hammer to the strings by hook or other technique. The technique used here is learned from Mr. Don Mannino, head technician at Kawai Piano. Push up the jack toes to let-off button with two fingers so the hammer blocks the strings (shown on picture in the middle). Plug each string to see if anyone is ringing or not being blocked by the hammer. If ringing, that means the surface under that string is low, and the high surface needs to be filed/sanded down. Mark which side to file/sand on the key stick, one octave at a time.
With Cheek blocks in, mark location of the action/keyboard. So we won't need to place the cheek blocks in and out over and over again. Each time sliding the action out and in, the front stops right at the marks.
File/sand on hammer tops according to the chock marks. The sanding paddles are homemade with fine sandpaper. The mid picture shows a tool with a thin strip of sandpaper fastened on a piece of plastic. It is easy to sand for single string contact spot. The plastic allows us to see where we are reducing. Push action back to piano, recheck hammer to string fitting. The same note may be adjusted over and over again till all three strings of each note are blocked by the lifted hammers. Wipe off the chock marks when mating is complete so we don't confuse ourselves.
All better. The piano was then tuned by the shop owner Mr. Jude Reveley before customer came to inspect. Thank you Ms. Steinway L. Your cooperation and company is truly appreciated.