Definitely do not wash the silk.  K&S is the old style shrinking type.  It shrinks nicely after the first wetting.  It can be applied dry, then water shrunk like Japanese tissue or applied wet.  If you do apply wet, you better have a strong structure!  It has been known to collapse stick and tissue free-flight models.

Once it has been wet and shrunk the first time, it only expands slightly when wetted and then returns to previous state.

RSM sells silk that is the more recent type.  You wet it, apply it as tight as you can, and then you are OK.  It expands slightly when wet, then shrinks back to the previous size.  If you buy his silk, READ THE DIRECTIONS!!!!  It works just fine if you do.