Most of us at some point have bought a used guitar only to find the guitar and/or case smells horrible. Sometimes it's smoke, sometimes mold/mildew. There are a million ideas on how to remove these smells. Some of the suggestions here I've tried, some I've found on forums, so do your own research!
First, the natural ones: Leave the case outside in the sun for a few days. Or leave a fan blowing directly at it to circulate air over it. Keep in mind some smells will take TIME to go away. It may be a long process.
Still in the natural realm, you can try:
Baking soda (sprinkle in and vacuum or leave in shallow tray)
Cedar or pine chips
Coffee (whole beans or ground)
White rice (sprinkled or in a sachet bag)
Dried oregano
Vanilla extract
Potato (baking type cut lengthwise)
(The potato I just thought was kinda funny, but a guy in a forum swore it worked.) Ok, still somewhat on the natural side: activated charcoal, the kind used in aquariums (don't use charcoal briquettes). Put it in some kind of porous bag. Lemon oil--sprinkle on a rag and leave in case or guitar sound hole. Some guys say to clean with distilled white vinegar. (Then air out to get rid of the vinegar smell!)
Still want more ideas? I saved these for last because these are chemical "masking" agents. Febreze--to me this smells bad itself, but some may like it. Supposedly there is an unscented version available. If you have the dreaded mildew smell, some say a few sprays of Lysol are worth a try.
Many swear by dryer sheets. I've found they work more for maintenance if you like their smell. I've read some good reviews on Zep Air & Fabric Odor Eliminator--cheap at Home Depot (UPDATE: I tried Zep and in my opinion, it is less stinky than Febreze and almost pleasant smelling). Some use carpet cleaners, especially with baking soda, but they have their own perfume smell.
Finally, people say ozone generators work, but they can be pricey, and they produce their own exhaust that can be harmful and must be aired out. Also, NEVER use perfume/cologne--worst smell ever in a case! Only some of the above methods work inside a guitar body. For instance small bags of coffee or cedar chips can be put into the soundhole, but vinegar or carpet cleaner are obviously not gonna work there. If you sprinkle baking soda or carpet cleaner directly in a case, you may want to turn the case over a couple times and/or play hand drum on it, to mix it around. Then be prepared to do a lot of vacuuming later. Now go get out that smell!
Thanks for these good ideas. Ground coffee should be helpful. I’ve also heard whole coffee beans, but not sure. Ozone generators are supposed to work, but you need to be careful and use them correctly with ventilation because they put out ozone which is harmful to breathe.
Oh, and Murphy’s Oil Soap is great stuff. I only recently discovered it and it works great to clean and revitalize rosewood fretboards. Has a pleasing smell, so I could see it really working to kill bad smells on a guitar. Thanks!
Murphy’s oil soap worked on my les paul and martin. The smell came from the cases. Don’t put back in case. The les paul actually absorbed the sweet smell from the case into the poly finish. washed it with sudsy murphy’s mixed with water on a cotton cloth, then rinse with water on another cloth. aired dried. I aired the les paul case out for over a month and the smell went away. The Martin case was old like it got wet in a river and then dried, I threw it away. Some of the chinese cases have a strong glue smell and have to be aired out open outside or in a shed. The murphy’s worked on both guitars.
how about putting a cup of grinded coffee in the case for a week or so? Or some tea bags. Maybe also humidify appropriately.
On a side note, you can make your own ozon generator for under 50 bucks, there are some nice youtube videos on that topic. You need a neon transformator from a neon sign, outputing anywhere between 3k to 12k volts.