Nitrate will shrink the stuff, but only a little. I use water to shrink, then seal it with nitrate, because it only takes a coat or two of nitrate to seal the tissue or silk, rather than several more of butyrate.

Good point about the durability of silkspan. You WILL be recovering your plane if you use silkspan over open bay framework. Over sheeted surfaces, no problem. It saves weight over silk. Over open bays, though, it pays to use silk. A little more weight, but MUCH more durable.