I have encountered the very same issue. In addition to the discomfort of wearing a sand-coated silnylon poncho, there is also the weight issue. That sand adds up! Since I do all of my backpacking in the Grand Canyon, sand is a huge issue. My solution has not been the discovery of some ingenious way to clean silnylon. Rather, I bought a Bozeman Spin poncho, which is actually lighter than a silnyon equivalent and made of silicon impregnated fabric that does not have the same sticky quality of silnylon. After one trip in the canyon, the sand accumulation problem was minimal, and it was much easier to brush off sand that did gather. It might be worth looking at, but prepare for sticker shock.