Symptoms
- Keys that stick, return slowly or feel heavier in summer
- Pitch drifting sharp and the piano going out of tune soon after a tuning
- Rust on strings, tuning pins or other metal parts
- A musty smell, mildew spots or swollen veneer on the case
- A muffled tone or a sluggish, less responsive touch
Likely causes
Swollen key bushings
The cloth bushings that guide each key absorb moisture and tighten around the guide pins, so keys stick or rise slowly.
Soundboard swelling
A damp soundboard pushes up against the strings and raises their tension, so the piano drifts sharp, unevenly across the range.
Corrosion of metal parts
Constant moisture, especially salty sea air, rusts strings and tuning pins. Rusty strings sound dull and break more easily during tuning.
Swollen action felts
Felts throughout the action and the hammers soak up moisture, adding friction and dulling the hammers' tone.
What you can safely check yourself
- 1Put a digital hygrometer next to the piano and check it morning and evening for a week; aim for 40–50% relative humidity.
- 2Run a dehumidifier or your air conditioner's dry mode in the piano room on humid days, with the doors and windows closed.
- 3Move the piano away from exterior walls, windows, the kitchen and the bathroom, and keep it out of direct sea breeze.
- 4Keep the lid and fallboard closed when you are not playing, to slow moisture getting in.
- 5Shine a flashlight inside on the strings and tuning pins and look for orange rust or white mildew.
What not to do
- Don't spray WD-40 or any oil into the action or on the strings; it ruins felt and attracts dirt.
- Don't dry the piano fast with a heater or hair dryer; rapid drying can crack the soundboard and loosen glue joints.
- Don't put rice, salt or moisture-absorbing containers inside the piano; they spill and can leak.
Quick fixes vs. call a pro
You can usually handle it
- Measuring humidity and running a dehumidifier
- Moving the piano to an inside wall
- Keeping the lid closed and wiping the case dry
- A single key that sticks only on very humid days
Call a technician
- Several keys sticking or returning slowly
- Visible rust on strings or tuning pins
- Mildew inside the case or on the hammers
- A piano that won't hold a tuning for more than a few weeks
What a technician will do
A technician eases swollen key bushings, frees sluggish action parts and checks strings and pins for corrosion; easing sticky keys usually takes 1–2 hours. Where the room is hard to control, a built-in humidity control system such as Dampp-Chaser keeps the piano itself at a stable level. Once the humidity is stable, a tuning will last much longer.
A piano holds about 18–20 tonnes of string tension. If a fix means opening the action, turning tuning pins or working near the strings, leave it to a technician.
In Israel
In coastal cities such as Tel Aviv, Haifa, Netanya and Ashdod, relative humidity is often above 70% through the summer, and sea air speeds up rust. An apartment that is closed up and not air-conditioned for weeks, for example during a summer trip, is the typical place where we find sticky keys and mildew.