Use masonry solicone?
I went thru all 14 pages in this thread, and didn't see anything about using "Brick & Tile Silicone Sealer". It's used for sealing porous tile or brick for exterior use.
The stuff comes in gallons. I could imagine dumping it into a bucket, stuffing in the finished tarp, and letting it sit overnight. Next day, hang the tarp up to dry, and pour the excess sealer back into the original container.
Anybody tried this?