rubberized undercoating is crap. when it stops sticking (which it will) then it will trap saltwater behind it against the metal.
my buddy steve bought an undercoating gun from Eastwood. he threw away the undercoating crap and modified the gun to accept a mobil1 bottle. mix up some 40wt non detergent oil from tractor supply and some motor honey. it goes on like snot and you have to adjust the air pressure pretty low to get it to work. add some kerosene to thin it if need be. works AWESOME!
Brian