What spray product could protect paper against grease?
I don't know whether you have finished all of your origami work, but I was just introduced to clear gesso. With this movement of Bible journaling, one of the big problems that they were having was either the drawings bleeding through to the next page or the pages were sticking together with decoupage. They discovered that a layer of clear gesso down on the page first prevented the bleeding and staining problems. This would be a solution to use prior to starting your project though.