The reason I need a high molar urea is to strip a nitrocellulose membrane over night but the high concentration urea kept precipitating after couple of hours.
I think the answer was air. As long as the high concentration urea (I tried 9 M) do not get in contact with air, it is ok even in a low temperature for a long time. I left it overnight and there is still no precipitation as I am typing. (Almost 24 hours by now)
Take a look at the picture below. I sealed it in a bag and let it rocking on a platform. No precipitation what so ever!!