Just a reminder that off-the-shelf shellac comes in 2 hues. There's orange shellac as well as clear shellac (somewhat yellowish). The former would be less likely to change the color of a brown hat whereas the clear version would be better for other colors.
I have used the off the shelf Kahl in a pump spray bottle. It is very clear and normally will be fine to use, HOWEVER before anyone uses any stiffener, sponge and brush your hat like crazy! If not, you will trap anything on the surface of the hat under the spray....not good! Also any time you spray a hat, do not rush anything. Hat stiffener is meant to be applies a small light application until dry and then again, until you get it where you want it. To rush is mess, I know I currently have a super nice fedora that I soaked with the spray....not smart. Now the brim is splotchy and hard as a board! I may have to ship it to a hatter and have Tonyb work his magic on it over to Tumwater Hats.