Purity: Removing Obstacles Before Ghusl
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 376:
Anything that is an obstacle for water to reach the body must be removed; if one performs ghusl before becoming confident that the obstacle has been removed, the ghusl is invalid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)