Can I use chametz (it contains denatured alcohol) hairspray on a shaitel before pesach if I will wear it on pesach?
Yes, this is fine.
Hairspray is not food, and can in principle be used even on Pesach.
Although some are stringent for this on Pesach (out of concern that it can be separated into alcohol), there is certainly no problem once it is sprayed onto hair.
Best wishes and Chag Kasher Ve-Same’ach.