Quite a few have asked me about our zippers and what to use when to going gets tough. My response based on advise given to me by a tailor is WAX. But, it seems no one believes me and they end up using some sort of lubricant which attracts dust and crap. I have found a video that supports my theory. One in which we practice too.
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Advgrrl Motorcycle Adventures & More 
I use a bar of soap. It’ll wash out the next time you wash the item.
wax lasts longer…just saying 😉
Been using candles on my zippers for years. It was a trick my dad showed me from his trapping days. They would wax everything to keep it going in dirt dust or freezing and they are right it works great, lasts a long time
wax on wax off! LOL