I'm not sure why you assume you should use a poly batting. I use W&N for all my wall quilts except those made for show and for those I use wool. W&N hangs better, flatter, and doesn't sag like poly. If you want it clean, but not puckery, I'd probably hand wash the top in cold water when it's done but before sandwiching and quilting. You could also pre-wash the batting, but only if you decide to wash the piece after quilting.