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Humidity and frequent exposure to moisture can make some bag or raincoat zippers harder to move.
For a non-waterproof zipper that's simply sticking—not damaged—lightly rubbing a dry bar of soap along the zipper teeth can provide temporary lubrication.
Move the slider gently afterward instead of forcing it.
Don't use this on specialized waterproof zippers without checking the manufacturer's care instructions, since they may require specific lubricants.
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