Maybe you should test it. I remember silnylon, 1.3oz, to be "waterproof" in a foot of water. Sink some in the sink and see what it does.
I sometimes buy mystery nylons at Walmart for making stuff sacks, kites, ground cloths, etc. You can probably ebay it off or sell here if you don't want it, but is sound like useful fabric. At a buck a yard, consider it a heck of a deal, no matter.