Purity: Removing Obstructions in Tayammum
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 696:
One must remove any rings from his fingers when wiping over his hands; and if there is any obstruction on his forehead, the back of his hands, or on his palms – for example, something is stuck on them – he must remove it.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)