Hello, just found this today hopefully I can still lend a hand. Coating foam in urethane or a like product could result in melting your foam, or something equally damaging, I have researched this as well and haven't found to many good results. But there are some other ways. First one is a method I just learned about, covering your foam in worbla, it's fairly new but I've seen some great results, not entirely sure how it's done but I heard it's fairly easy and uses tools you would use for foam armour. Along the same line is wonderflex, it's a older version of the worbla method. Last is xrobots from the UK, a YouTube search or his website has a method of coating foam in some kind of plastic, think hardcore plasti-dip.
Hope this helps, cheers