Purity: Discovery of an Obstruction After Wudu
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 296:
If after performing wuḍūʾ someone sees on a part of the body on which wuḍūʾ is performed something that is an obstruction to water reaching that part, and if he does not know whether it was present at the time of performing wuḍūʾ or it appeared afterwards, his wuḍūʾ is valid. However, if he knows that at the time of performing wuḍūʾ he was not aware of the obstruction, then the recommended precaution is that he should perform wuḍūʾ again.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)