Fill a spray bottle half-full of white vinegar and half-full of water. The ammonia in cat urine will be neutralized by the vinegar's acidity. Allow this solution to sit for 30 minutes after saturating any areas that still smell strongly of old cat urine. After letting the solution sit for a while, use a paper towel to absorb any remaining moisture. If the old cat urine is still present, respray the area and allow it to dry overnight.