> In both cases the basic material (nylon) has had a coating applied to
> it. The ENO fly is coated with polyurethane and the silnyl flys have
> been treated with silicone. Polyurethane is a heavier (more weight to
> carry) coating than silicone coated.
> Anything that has seams (in either material) will need seamsealing to
> prevent the stitching lines from leaking.
> I like silnyl better because it is lighter. However, it can be a little
> more fragile than other materials. It is also noisier unless pitched taut.
Polyurethane is a coating and will eventually peel off. Silnylon is nylon impregnated with silicone and won't peel.
Silnylon does stretch/sag a little, which is why people are talking about tarp tensioners. For the ultimate lightweight tarp without sag, spinnaker would be the thing - no need for tarp tensioners, lighter than silnyl.
I'll stick with sil - lasts longer than PU, lighter by a magnitude, packs smaller than small, and under the conditions I camp in, it works fine to keep the rain off. Under some circumstances water can mist through it, but I haven't run into fine mist yet and likely won't.