Is undercoating good or bad ?

[olebluegmc]

I was planning on coating all my patch panels with 3m undercoating where it won't be getting welded, like my cab mounts, and rockers on the inside where it won't get welded. I Thought this would be good to prevent future rusting out of these troubled areas, however I was talking to a mechanic at work and he said undercoating causes more rust because the moisture can get trapped underneath and expedite rusting. Looking to see if anybody has had good or bad issues with undercoating ?