What are the parameters for the concession of pardoning a slight barrier in Wudu and Ghusl according to Shaykh al-Islam Ibn Taymiyyah? Is hardship a condition for its removal? Is it permissible to adopt this view when one becomes aware of it before the prayer, or is re-performance obligatory? Are there any other parameters?

It appears from Ibn Taymiyyah's words that he grants a general pardon without restriction. This is indicated by the fact that the correct view is the validity of purification (taharah (definition)) even if there is a small amount of dirt under the nails that prevents water from reaching them, especially for those who find it difficult to guard against it, such as workers in demanding trades. Ibn Taymiyyah analogized every small impediment that prevents water from reaching, whether it be blood, dough, or the like.