This has happened to me at least half a dozen times before but I never found a really great solution so this time when it happened I thought to ask here.
If I've stayed in one place for a while or been between trips for a while, my backpack and daypack get little attention in storage. Several times when I've next used them I've found that one or more zippers have seized shut. This happens especially if the backpacked was sweaty or damp when stored or when stored in salty air near the sea.
About half the time no matter how gentle I'm trying to be I end up breaking off at least the tab of the zipper. A few times I ended up breaking the whole zipper. And the rest of the time I manage to free it up without breaking anything.
Metal zippers tend to be made of cheap die-cast "pot metal" which is not very strong but can become brittle with corrosion.
I know I've tried penetrating oil like WD-40 and I've tried soap. I can't remember what else I've tried but they all seemed a bit hit-or-miss. Sometimes gently forcing it a tiny bit one when then the other and gradually increasing the movement works, but sometimes this ends up in breakage.
Is there a known product or technique that's much more likely to work most of the time?