Since lightweight waterproof synthetic fabrics are suddenly unsuitable for serious backpackers anymore, I thought "WWND?" (What would Nesmuk do?)
Recipe for waterproofing tent material (duck).
I have found nothing better for the purpose. "To ten quarts of water add ten ounces
of lime or four ounces of alum; let it stand until clear; fold the cloth snugly
and put it in another vessel; pour the solution on it and let it soak for twelve
hours; then rinse in luke-warm rain water, stretch, and dry in the sun." Cotton
duck treated in this manner has its capability of shedding water more than doubled.
From "Woodcraft" by George Sears