Sorry to inform you, the smell will never go away. We had a mouse get in the house and evidently it died inside one of our walls in a bathroom area. For over 4 yeas we had this hideous dead mouse/urine smell in that bathroom. Finally, we decided to remodel and expand the bathroom into another room and the entire 2 rooms were completely gutted. Guess what we found? Yep, the body, and WOW did it smell once the wall was opened up. Got it all cleaned out, bleached the "snot" out of the studs, and eventually sealed up the walls with new dry-wall and insulation. No more smells sense. (FYI...I also sealed those walls so tight, not even air can get into them now ... let alone a mouse!) Smell gone.
Good luck though with the cover up.