Well, you can get Permanent Water Guard by a company called Atsko, some WAHM's sell it, or do a search online. Or you can use a water proofing spray called Camp Dry sold at Walmart I think in the shoe section, or check in the camping/sporting section, they should have something there that is good for waterproofing synthetics.
I personally think felted wool flannel is the best to use, it's awesome. And you can lanolize it very easily, plus it's natural. LOL, although some babies have allergies to either fleece or wool...glad to help!
*hugs*
Amber