When you buy a used piano, you may be faced with the task of getting rid of its musty odor. Cigarette smoke, bug and rodent infestations and mold cause most of the piano odors. Pianos that give off a musty smell may have mold or mildew growing inside them. Remember, many pianos are over 30 years old and just because you found them in someone's living room, doesn't mean that they were never stored in a damp basement or dirty garage for a short period of time or even for years in some cases.
You can try removing the bottom panel located under the keyboard and vacuuming the dust out of the lower compartment. You may then place a new, opened box of baking soda in the lower right-hand side of the compartment. If you find that this helps, but doesn't completely solve the problem, call your local Piano Tuner-Technician and ask them to schedule a service call to perform an internal cleaning.
The tuner will come out to your home, take the piano apart and take all the keys out, exposing dust and mildew that has been trapped inside your piano since its first year. (I highly recommend that you DO NOT attempt to remove the keys yourself. You can severely damage your piano and it could end up costing hundreds of dollars to fix.)
The technician will then thoroughly vacuum and clean the internal cabinet of your piano the proper way. Cleaning solvents may be used in this procedure. They may also used a compressed air hose to get way inside the action and behind the strings. An opened box of baking soda should then be placed in the compartment. You may want to schedule a piano tuning also. Pianos should be tuned every six months. This procedure can cost anywhere from $125 to $250 depending on your tuner's rates and how dirty the inside of the piano is.
Your piano should be "ship-shape" after this procedure. In tune and without that musty odor, allowing the piano player to stay in "practice mode" for a longer period of time without breathing that unhealthy odor.