Musty Smell on a Sofa?

I'd loosen the tie downs/snaps etc. to have it ready to go outside on the next sunny warmer day for a good "sunning/airing out" on both sides. If the odor is not coming from old body oils, since it is used furniture, I'd suggest that it may be old cloth dyes. If it IS body oils, you may have to "baby powder" or "flour sifter" it in Baking Soda VERY well for a few days, one side powdered well and turned down to a well papered floor for a day, then VACUUMED OFF, bag emptied; then other side the same way.

If the odor is significantly reduced, plan on some sort of good mattress cover that you can BLEACH on occasion, if possible, in case the mattress is full of bacteria, which is very common if the previous owner failed to use a mattress cover. If you have no mattress cover, use a doubled sheet that you can bleach, keeping in mind to pay attention to cracks,

deep seams, edges, and feet/head/mouth areas
can be separately sprayed with alcohol until saturated, then blow dried. All folks drool at some point in their sleep life. Drool has odors. Many children wet the bed, which can eventually over many years of storage/ disuse, smell very musty, and may not clean out or be removeable. Just keep in mind, also, not to put water on it because it will likely circle and not be as effective. Sofa sleeper mattresses are usually not designed to last too long unless terribly expensive, so if you keep watching for someone tossing a sofasleeper, you might check to compare their mattress with your own and switch with theirs?

Good luck and God bless you in getting to the "bottom of it". ! LOL : )

Thanks so much for your advice on the sofa. As I type this the mattress is on the back deck getting a bit of sun! It does not smell as strong today as it did when we first got it home. Thanks again for all of your good advice.

Laurie