Purity: Dirt and Residue Not Obstructing Water
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 293:
If a place on the body that must be washed or wiped is dirty but the dirt is not an obstruction to water reaching the body, there is no problem. Similarly, there is no problem if after plastering and suchlike, something white remains on one’s hand that does not prevent water from reaching the skin. However, if one doubts whether or not its presence will prevent water from reaching that part of the body, he must remove it.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)