I'm not really the guy to ask. However, my thought on it would be "what is this guy trying to hide under there?". Also I would think that if ANY water is remaining in the frame parts, then rust could start up under the coating.
But your car, your way.
Actually I purposely did nothing under my car from the engine bay back. I left it totally as it is, dirty, but no rust or other nasties under there.
EDIT: I remember now that there is a product call Lizard Skin that is supposed to be really good , but again prep is king.