Painful, like a hot, burny sensation? I'd make sure the upper lip is well shaven, dry and clean, free of oils. The burny sensation is supposed to subside after the appliance is bonded. But if your skin might be too sensitive for spirit gum, I'd recommend trying Pros-Aide or any other medical-grade adhesive, it's water-based and not as irritating as spirit gum.
Of course, if Pros-Aide isn't in the cards, I'd try recommending using liquid latex (if you have any) if it's any better!
