This posts lists suppliers of “wax” concoctions used to coat inseaming threads so they stick in place and resist moisture.
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Panhandle Leather
Sorrell Notions & Findings
Starko Tools
It gives the thread water-repellent properties, the flax fibers stick together and this makes the thread stronger against tearing. Contains natural ingredients such as bee products and resins of coniferous trees, prevents the appearance of fungus and mold. Sticks to the thread and not to the hands.