I think I remember Cary and/or Craig saying that an undercoating would worsen problems caused by condensation?? I will search the forum threads to see if I can find that explanation and post it here if I do.
EDIT: Here is the thread I was thinking of. It talks about being sure to keep the edges of the vertical sidewalls sealed, as that is where water will be absorbed and do potential damage. Undercoating/sealant discussion