Slightly off topic but please don't ban me.

All this talk of coatings reminds me that I think we had a thread a while back with a video showing someone doing a do-it-yourself silnylon tarp. IIRC, they took the nylon fabric they had purchased, had the silicon waiting in a bucket, and slowly put the nylon into the bucket to be literally soaked in the silicon. He then pulled it out and hung it to dry. Perhaps a similar process may work with the polyester poncho, though the other suggestions from doug and art sound a whole lot simpler and easier. And I am wondering if the simpler processes might work to make your own silnylon as well.

So much research, so little time.