TL;DR
Piano keys are cleaned with a cloth wrung out so thoroughly that it feels barely damp, because water running down between the keys reaches the wood and felt underneath and causes real damage. Wipe front to back, one key at a time, dry immediately, and never use alcohol, bleach, or any household cleaner on a keyboard.
What is under the keys
A piano key is a long wooden lever with a covering on the visible end. Behind the part you see, that lever runs back into the action, a mechanism of wood, felt, leather, and cloth bushings.
Water that runs down between two keys goes there. Wood swells, felt compresses and mats, and bushings tighten until keys stick or stop returning.
That is why every piece of piano cleaning advice is built around using almost no moisture at all.
How do you clean piano keys?
Dampen a soft cloth with water and wring it until it feels barely damp. Wipe each key front to back, from the back of the key toward you, one key at a time, then dry it immediately with a second soft cloth. Never let liquid run into the gaps between keys.
Ivory and plastic are different
Plastic keys are on virtually every piano made in recent decades. They tolerate a barely damp cloth well, and a tiny amount of mild dish soap in the water is acceptable for stubborn grime. Rinse by wiping with a second barely damp cloth and dry immediately.
Ivory keys are on older instruments, and ivory is porous natural material that absorbs moisture and stains. It uses even less water than plastic, and never soap solutions that could soak in.
Ivory yellows naturally with age from oxidation and from being kept closed away from light. That is not dirt, and it will not clean off. Some ivory can be lightened by a technician, but it is a restoration job rather than a cleaning one, and heavy-handed attempts damage the keytops permanently.
There is one persistent piece of bad advice worth naming: do not put toothpaste, milk, or a bleach solution on ivory keys. Toothpaste is abrasive and dulls the surface, milk leaves organic residue that will smell, and bleach damages ivory and anything it runs into.
Tell ivory from plastic by looking for a fine grain pattern in the surface and, on many older pianos, a visible seam where the keytop is made in two pieces. A technician can confirm it.
Why do piano keys turn yellow?
Ivory yellows with age through natural oxidation, and it yellows faster when a piano is kept closed and out of light. Plastic keys yellow from ultraviolet exposure, skin oil, and smoke residue. Neither is dirt, and neither reliably comes off with cleaning.
For plastic keys, a barely damp cloth with a drop of mild soap removes the surface layer of grime that makes yellowing look worse.
What never goes on a keyboard
Alcohol. It dries out and can craze both ivory and older plastic keytops.
Bleach. Damages keytops and anything underneath.
Ammonia-based glass cleaner. Attacks finishes and plastics.
Furniture polish. Silicone contamination is as unwelcome on a piano as on a guitar, described in how to clean a guitar body and strings.
Abrasives of any kind, including scrubbing pads and toothpaste.
Spray anything. Never spray a liquid at a keyboard. Anything that goes on the cloth goes on the cloth first.
A soaking cloth. If you can squeeze a drop out of it, it is too wet.
The keyboard routine
Dust with a dry microfiber cloth or a soft brush before anything damp, so grit is not dragged across the surface.
Wipe front to back, never side to side. Side-to-side wiping pushes moisture and dirt into the gaps between keys.
Do one key at a time on the white keys, then the black keys, drying as you go.
Black keys on older pianos may be ebony or a stained hardwood, and those take even less moisture than plastic. Dry immediately.
Wash your hands before playing. Skin oil is the source of most keyboard grime, and hands are the whole problem.
The rest of the instrument
The case finish depends on what it is. A high-polish polyester finish takes a soft dry cloth and, occasionally, a polish made for that finish. A satin or hand-rubbed lacquer finish must never be polished, since polish leaves permanent shiny patches.
Do not use household furniture polish on a piano case for the same silicone reason.
Dust the case with a clean, dry microfiber cloth, working gently. Grit under a cloth scratches a gloss finish.
Inside the piano is a technician's job. Do not vacuum the strings, the soundboard, or the action yourself. Dampers, felts, and hammers are easily damaged, and strings and tuning pins are not something to poke at.
Under a grand piano and behind an upright collect dust that ends up inside. Vacuum the floor around and under the instrument regularly. That is the part you can do safely.
Source: Piano Technicians Guild care guidance for piano owners
The environment matters more than cleaning
Pianos are made mostly of wood, and wood responds to humidity.
The usual recommendation is a stable relative humidity in the range of 40 to 50 percent, and the stability matters more than the exact number. Swings are what crack soundboards and loosen tuning.
Keep a piano away from exterior walls, heat vents, fireplaces, and direct sunlight. A room that swings between damp winter and dry heated air is hard on an instrument, which is a real consideration in an Oregon house.
Close the fallboard, the cover over the keys, when the piano is not in use to keep dust off. On instruments with ivory, some technicians suggest occasional light exposure to slow yellowing, so ask yours.
Regular tuning is maintenance, not cleaning, and most technicians recommend once or twice a year.
When to call a technician
Sticking keys, keys that do not return, uneven key height, buzzing, and anything that changed after a spill.
Yellowed or chipped ivory that you want restored.
Anything requiring the action to come out of the piano.
A spilled drink is an emergency. Do not tip a piano, do not pour anything into it, and call a technician promptly, since felt and wood damage worsens as it dries.
The same do-not-improvise principle applies to other instruments in how to clean a brass instrument mouthpiece and across our guide to cleaning tools and equipment around the house.
FAQ
Can you use water to clean piano keys?
Only a cloth wrung out until it feels barely damp, wiped front to back on one key at a time and dried immediately. Water running into the gaps between keys reaches the wooden levers and felt bushings underneath, causing swelling, sticking keys, and matted felt that a technician has to repair.
How do you whiten yellowed piano keys?
Usually you cannot, and you should not try with household products. Ivory yellows through natural oxidation and plastic yellows from light and skin oil, neither of which is dirt. A barely damp cloth removes surface grime that makes it look worse. Ivory restoration is a technician's job.
Is it safe to use disinfecting wipes on piano keys?
Not on ivory, and cautiously at best on plastic. Most wipes contain alcohol or other actives that dry out and craze keytops, and the liquid can run between keys. If keys must be cleaned between players, a barely damp cloth with a trace of mild soap, dried immediately, is safer.
How do you clean ivory piano keys?
With a cloth wrung nearly dry, wiped front to back, one key at a time, and dried at once. Ivory is porous and absorbs moisture and stains, so use even less water than on plastic and no soap solutions. Never use toothpaste, milk, bleach, or any abrasive on ivory.
Can you polish a piano case?
Only if you know the finish. A high-polish polyester finish takes a polish made for it. Satin and hand-rubbed lacquer finishes must never be polished, since polish leaves permanent shiny patches. Avoid household furniture polish entirely, because silicone in it complicates any future refinishing.
How often should piano keys be cleaned?
Dust with a dry cloth weekly, and do a barely damp wipe every month or two, more often if the piano gets heavy use. Washing hands before playing prevents most of the buildup. Closing the fallboard between sessions keeps dust off and cuts the cleaning need considerably.
Instruments sit in rooms that need their own upkeep. If the rest of the house is the harder job, get a quote or read more about house cleaning services.