Answered by Sidi Moustafa Elqabbany
If, without doubt, najasa (such as urine) falls on our garment and then dries before being cleaned, what are we to do?
Answer:
In the Name of Allah, Most Merciful and Compassionate. All praise is due to Allah, Lord of all things. May Allah bless and grant peace to our Master Muhammad, his family, and companions.
First, remove all traces of the filth using whatever means necessary (e.g. scrubbing, wringing, detergent), then run plain water over the entire area. [Reliance, e14.10-11; Nayl-ur-Raja, p. 53]
The above assumes that the filth is not from dogs or pigs.
Allah knows best and He alone gives success.
Moustafa Elqabbany
Metro Vancouver, Canada