Answered by Shaykh Hamza Karamali, SunniPath Academy Teacher
I would just like to know if someone has applied hair gel to his hair, may he make wudu over it or not?
Answer:
In the Name of Allah, Most Gracious, Most Merciful
In the Name of Allah, Most Merciful and Compassionate
assalamu `alaykum wa rahmatullah
The General Answer
The water that one uses in wudu must actually reach the areas that it is meant for. If there is a barrier that prevents the water from reaching the one of the limbs that one washes or the hair that one wipes, then it is as if one has not performed the ablution at all.
Hair gel solidifies after application to form a solid layer that does, in fact, prevent water from reaching the hair underneath. If one every single hair that one wipes over during ablution has a layer of solidified hair gel on it, then the ablution is not valid since one has not wiped any hair at all. If, however, one is confident that one has wiped at least some hair that were not covered by a layer of hair gel, then the ablution is valid.
One way out of this problem is to leave a small portion of one’s hair without any hair gel on it and to wipe over this part when performing ablution. In the Shafi`i school, wiping over the slightest part of the head or hair will result in a valid ablution (although it is superior to wipe the entire head).
Important Note #1
The hair that one wipes must naturally lie within the limits of the head. It is not valid, for example, to wipe the part of one’s hair that naturally falls onto one’s forehead, neck, or shoulders. One would have to wipe the part of this hair that lies closer to the roots within the limits of the head.
Important Note #2
Many kinds of hair gel contain alcohol (i.e., ethanol), which is impure. It is not permissible to apply impure substances to the body and to perform the prayer with an impure substance on one’s body or clothes is not valid. If one does use hair gel, one must seek out a brand that does not contain any impure substances.
And Allah Most High knows best.
Hamza.
islamqa.org/shafii/qibla-shafii/33258 
