All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
You should know that if a person does his best to remove all that prevents the water from reaching the skin and then something tiny remained preventing the water from reaching the skin, then this is excused. It is stated in Ar-Rawdh Al-Murbi' (a Hanbali book) with its Haashiyah that "small [amounts of] dirt under a fingernail and other similar cases such as something inside the nose that prevents the water from reaching the skin do not affect purity. Rather, the purity of such a person is valid, and this is the view chosen by "Al-Muwaffaq" (Ibn Qudaamah) and others, and this is the view that is considered as the correct one by Al-Mardaawi, the author of Al-Insaaf. The Shaykh (Ibn Taymiyyah) said: 'Any tiny dirt in any part of the body or dirt in cuts in the feet is excused, and the same thing applies to any tiny thing from the body that prevents water from reaching the skin, like blood, dough and the like.' This is the view chosen by him (Ibn Taymiyyah).” [End of quote]
Shaykh Ibn ‘Uthaymeen said: “Shaykh Ibn Taymiyyah is of the view that the legal reasoning is based on hardship; whenever it is difficult to avoid impurity, then a small quantity thereof is excused. It is the same thing regarding painters, small paint is excused if it is on their body and prevents water from reaching the skin, because religion is easy, and such matters happen very often to the person, while sometimes he does not feel it or he feels it but it is difficult for him to avoid it.” [End of quote].
For more information, please refer to Fatwa 130050.
Allaah Knows best.