I would guess that if your in the north salt spray is going to get on them no matter what. We don't have that problem in South Texas but I do spray everything underneath with a spray bed liner product from rustolem as we do get salt spray while on the ferry at the coast and salt spray in storms when camping in one particular RV park...called On The Beach...it's close.
2015 338RETS Eagle Premier gloss sides., king.
2012 Ram 3500, crew cab 4x4 duelly Deleted
16k miles, 530HP, 1000 +torque.