We finally attempted - again - to unfreeze some headliner zippers this week. Have tried in years past with products that claimed they'd do the job. Literally every zipper on the boat was unusable, and we only succeeded in getting one to open four years ago. Well this week, on the advice of a fellow sailor we used this:
Free All. And by God, it actually worked. Gave it a good soaking, let it sit for about 40 minutes, and were able to get it working. Cannot wait to replace all the old broken/useless sliders, and gain much needed access above the headliner without having to remove it.