As long as your careful and only touch the metal zipper when it's done up, I always use the WD40 pen. Sounds really awful and stinky but, it's so handy to have this around the house for things like this. If I use this method to unstick a stuck zipper, I always make sure to put paper towel behind where I'm working, (folded up in case I go a little nuts) to soak up any excess and make sure I'm ready to go. And the great thing about this method is that it's completely colourless and the pens are pretty much scent-free.
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