Old urine stains have an odour that is particularly difficult to eliminate. Most cleaning products mask the odour for a few weeks, but it will return, usually as strong as before. An enzyme-based cleaner is needed to get rid of lingering urine smells. Eliminating urine smells is particularly important if you have pets, as they tend to urinate over top of old urine smells, making the problem an ongoing issue.
Wet a sponge with plain water so it is damp but not dripping. Sponge water onto the urine stain. Press a towel on the stain to draw the moisture back out. This will help remove some of the urine and any cleaners that were previously used.
Spray the stain with an enzyme cleaner, which is designed specifically for consuming proteins in urine. Find them at pet supply stores and follow the manufacturer's directions. Some require rinsing, while others need to be left on the stain.
Dry the mattress thoroughly. Open a window or point a fan at the stain. You want the mattress to dry quickly and thoroughly to prevent it from growing mould.
Use a black light to find urine stains.