Is one allowed to use hairspray on shabbos? Let's say a braid was made before shabbos and one wants to spray the braid on shabbos to prevent it from getting messy.
For any practical application, you should ask your rav as this is an issue where poskim (halachic decisors) differ. R Eliezer Melamed forbids it
One may not use hair spray or oil to keep one’s hair in place, because this is similar to Boneh (Rivash as cited in BHL 303:27 s.v. “laĥof”).
Matzav.com brings a more nuanced perspective though
The use of hairspray which will stiffen the hair and form a structure may involve the Melocho of Boneh (building). If one sprays the hair first and then styles and shapes it; this would be prohibited on Shabbos because it is similar to Boneh.
However, some poskim permit the use of hairspray if one is careful to first style and shape the hair, and then only afterwards add the hairspray. When done in this manner the loose hair is shaped at the time of styling, and does not resemble Boneh because the hair is loose. The subsequent spraying merely acts to retain a pre-existing shape. Once the hair has been sprayed it can no longer be shaped, styled, or touched.
Shulchan Aruch w/Mishnah Brurah 302, Shmiras Shabbos Kehilchasa 14:50:131, Kitzur Hilchos Shabbos re:Gozez:5:16, Biur Halacha 303:27, Hilchos Shabbos R’ Shimon Eider, Minchas Yitzchok 6:26, Sefer 39 Melochos
Halachafortoday (651) brings a similar approach from the Mishna Brura
If you style [the hair] first and then afterwards spray a little hairspray, it is permitted according to many Poskim, as the spray is then only maintaining the hair’s shape. Spraying first, however, and then styling it is prohibited. (See [Mishna Brura] Biur Halacha 303:27 Dibur Hamaschil Lachuf)
So please CYLOR - hoping the above sources will help him come to the right decision.