My guess is that hairspray residue is causing problems with the electrodes. Try using butane as a propellant. It burns much cleaner. You should be able to find these at the neighborhood drugstore for lighter refills. Getting a good, consistent fuel-air mixture is going to be the tricky part, but that is true for any combustible gasses.
I'm not sure which grill starter you are using, but piezoelectics should do the trick. You can canabalize an electric lighter for this part. They tend to easily arc over a 1/8" gap.
Next, you should look at your projectile. How accurate can a potato be? Perhaps you should be firing well formed speckknoedel instead
