just unscrew the end cap, spray in some hair spray, cap it and hit the button. we were using a push button bbq starter for an igniter.
the one thing i hate about hair spray is it gums everything up. if you don't clean the threads when you're done that stuff acts like glue on the end cap. that's one reason why we switched to using acetylene most of the time. we used schedule 40 pvc and never had any issues with breakage.
btw, if anyone decides to make one of these check your local laws. they're considered firearms in some places so you could get in trouble. never, never, never, never shoot someone with one. you could do a lot of damage. i shot my friend in the right ass cheek from about 30' and he's very lucky i didn't hit him anywhere else (there was a reason, i wasn't just being an asshole). when it hit him, he dropped and just layed there. he showed me his ass cheek at work the next day and the whole thing was purple. he could barely even sit down.
potato guns are not toys and you can seriously hurt someone or yourself if you aren't careful.