I would like to ask about silicon, which is a substance that covers the hair like plastic, and it is waterproof. Do I have to remove it from the hair? What is the ruling on my prayer right now, as it disappears after one month? Do I have to check the shampoo before using it to see whether it contains waterproof substances?
There is nothing wrong with using silicon in hair treatments if it is free of harmful effects.
It is not necessary to remove it when doing wudoo’; rather that is only required when doing ghusl following menses or in the case of janaabah (impurity following sexual activity).
That is because wudoo’ is based on leniency, and what is required is to wipe over the hair only, and not wash it. Hence it is permissible to wipe over the turban, or over a woman’s hijab. The Prophet (blessings and peace of Allah be upon him) stuck his hair together during Hajj in a process called talbeed, when the hair is stuck together using gum and the like, which creates a layer over the hair and may prevent water from reaching it.
But in the case of ghusl, it is stipulated that the water should reach all of the hair and the scalp, so it is essential to remove this substance when doing ghusl.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: It is not disallowed for the muhrim (pilgrim in ihram) to use gum or honey to stick his hair together (talbeed). In other words, there is nothing wrong with him putting gum and honey on his hair, to keep it from becoming dishevelled.
It is well-known that if he does that, then this gum and honey will prevent water from touching the hair, but there is nothing wrong with that. Hence it is permissible (in the case of wudoo’) to wipe over the turban even though it prevents the water from reaching the head, and that concession is allowed only with regard to the head, and not the hands, face or feet, because the basic principle is lenience with regard to purifying the head, and all that is required is to wipe it.
Based on that, we say:
If a woman has stuck her hair together with henna, can she wipe over it when doing wudoo’, or do we say that it is essential for her to remove the henna? There is nothing wrong with her wiping over it, even if there is henna on it that prevents the water from reaching the hair. End quote from Sharh al-Kaadi.
He (may Allah have mercy on him) was also asked: What is the ruling on wiping over henna on the hair when doing wudoo’?
He replied: There is nothing wrong with it, even if it prevents water from reaching the hair, but when doing ghusl in the case of janaabah or following menses, it is essential to remove it.
The fact that there is nothing wrong with wiping over it when doing wudoo’ is indicated by the report that when the Prophet (blessings and peace of Allah be upon him) was in ihram for his Hajj, he had stuck his hair together with gum or honey and the like (talbeed), so as to avoid becoming dishevelled. When it was said to him: O Messenger of Allah, are you not going to cut your hair – i.e., following ‘umrah – and exit ihram as the people do? He said: “I have brought my sacrificial animal, and stuck my hair together (talbeed), so I will not exit ihram until I offer my sacrifice.”
Therefore there is nothing wrong with having henna on the hair, even if it prevents water reaching the hair, when doing wudoo’.
But when doing ghusl in the case of janaabah or following menses, it must be removed.
End quote from al-Liqaa’ ash-Shahri (68/14).
See also the answer to question no. 198392.
And Allah knows best.