I've got a standalone building in my back yard that I used for storage
and left closed up for over a year. Now it has that characteristic
musty storage odor to it, and I can't get it out for anything.

This storage room is like a standalone room in a house. It has wood
paneling and a laminate floor. It's got windows, a door, and a small
built-in air conditioner. It's about 11 x 19.

I've tried a window fan running 24 hours, and also tried an ionic air
cleaner, but as soon as the unit goes off, the smell comes back. I've
used a kind of "lava rock" odor absorber, which proved worthless, and
even tried wiping down the walls with dryer sheets.

Do I have to tear out the paneling to get rid of the smell? Would a
cubic yard of activated charcoal get rid of the smell forever? Any
ideas are appreciated.