I am working as a piano technician and started rebuilding/repairing pianos. I am not doing complete rebuilds, but replace and repair only what is necessary to have a very decent result for a low price.

Up to now, I worked on different type of hammers, I mean different type of brands and felt. Voicing worked fine and I also used hammer hardeners in some cases to improve the sound of treble hammers and hammers in the tenor transition.

Now I have an upright with very soft hammer felt. The sound in the tenor and treble is soft but very appealing. The sound in the bass although is very soft and lacks power.

Battery voicing helped a little, but I think the felt lacks tension and the possibility of battery voicing might be limited. I did not try exessive battery voicing, though. Until now, I only used some stitches, maybe 5 to 10 per hammer.

How would you treat these hammer? Would hammer hardener help? What regions should be soaked with hardener and how deep?

I still would like to have a decent tonal range. Now the sound is mostly soft. My fear is, that using hammer hardeners the sound might be too hard?

Looking forward to your opinions

Upright

I may be the only one to suggest this. Try it on something that won't matter first. The only place I have seen this in print is in Dr. William Braid White’s book.

Take a hammer iron and get it too hot. Scorch the outer layer of felt. Then use a small hand wire brush to take the char off and finish with a sandpaper paddle. The layer that is now exposed will be tighter and the tone will be brighter and more "focused". I have had good success when inadvertently over-needling hammers.

You can try this on a 1cm x 1cm x 5cm scrap piece of hammer felt to see what I mean. Do this to one side of the scrap that is layered, and then bend it one way and then the other. It should be obvious that the layer beneath what was scorched is tighter.

Make sure hammers are filed and finish up with 600 grit. Try one drop of acetone and keytop on the strike point. If you like the direction it goes, you can always add more. Nothing wrong with a hammer iron, if not over done but I rarely use one these days.

Don't go too far, or the middle and treble will sound too soft in relation. Everything is relative. By middle c, I'd want the tone to be climbing brighter into the treble, not soft, so you might want to add juice starting about C4 and into the treble, given your description.

Inserting a voicing needle into different parts of a hammer will give you an idea what's going on, but interpreting that takes experience.

Re-reading this post gave me reason to pause. So this is a refurbishing job. These are not new hammers. And these are old strings.

Let's back up a bit. Why do you say this piano has very soft hammer felt? It may, or it may have dead bass strings. Can you tell us more about the piano? How old, how big, have the hammers been reshaped, have the butts been repinned, how loose is the pinning?

The piano is a small Wolfframm, that is an old brand from Dresden, Germany. It is from the 40s. The strings are is very good condition and the key tops are plastic, so I assume it was refurbished previously. The hammers look original.

The hammers had signs of moderate use and I did the filing/reshaping myself. Considering the amount of felt, that is still there, they have been reshaped previously more than once.

The felt in the treble section is firm enough and this small piano has a long and nice sounding sustain. The middle and the bass section seems to be needled down. The shoulders are very soft. The needles go easily in there, when I do some test stitches. The strike zone is a little more firm.

I thought of lacquering the shoulders to give the hammer more support. But I am a total newcomer to lacquering. I have an acetone based hammer hardener liquid from a german piano parts supplier, that I used successfully on Renner hammers in another upright in the trebble. But I did not use lacquer extensively yet and not in the bass section.

There is not a lot to loose, as I got the upright for little money and if my improvement attempt fails, I may replace the hammers. There is no customer waiting for it yet. But I am also keen on learning and thought it might be a good idea to discuss this here.

Upright

Upright:

If there is little difference in tone between the solid and wound strings, and there is obviously a soft feel to the hammer felt when applying needles, AND the hammer butt pinning if firm (you had not said one way or the other) then, yes, hardening the hammers in some way seems to be a good idea.

You will get many suggestions on how to do so.

Try scorching a scrap and see what you think.

I"m with UprightTooner on this one. I wouldn't touch the hammers before closely checking the bridge and replacing the bass strings.

I will probably replace the bass strings. Some have a sizzle in FFF and turning did not help. The bridge looks ok for me. Most probably I will replace the hammers too. Right now I am practicing the effect of lacquering in different regions.

@UnrightTooner:
The pinning is firm but not too firm. Seems right to me. So I started using hardener. I started with the top and the shoulders. This opened up the sound, but only little improved the power. I am testing hardening at the base now.

What about hammer to string alignment? Bi-chords could be affected. No power. Let-off? Regulation?