That’s a solid idea about using the screw-off cap to spray hairspray directly into the back end of the cannon. To make sure the hairspray stays contained, you could also try using a rubber seal or gasket around the cap to prevent any leakage. Another suggestion is to lightly tape or seal the opening...

To clarify, if you're having trouble with the hairspray escaping while loading the potato, maybe you could try sealing off the back of the cannon more securely before you spray the hairspray. You could try using a more airtight plug or cap while loading the potato, then remove it when you're ready t...