The covers on my cabin cushions are dirty. They seem to be in good shape, just in need of dry cleaning. So I thought I would take off the covers and send them to the cleaners. My problem is that the zippers are stuck. They zipper itself appears to be nylon, but the zipper pull mechanism is metal and appears corroded. I tried a little WD40 and a little Teflon/Silicone spray, but they aren't getting freed. Any suggestions?Thanks!