i could be tottally wrong,, this is just my opinion,,,but all that undercoating from back in the day,,i think helped hold in the moisture,,and promote rust and rot,,,remember rusty jones undercoating back in the 1970s & 1980S,, ??? where did they go and look at all those cars that rotted,,,,ide clean it up and paint it with some thing like POR 15 and be done wit hit