On my Grand I sprayed Amsoil MPHD (heavy duty metal protector) on the underside. A friend of mine did it to a 2008 Wrangler and has no rust after 4 salty midwestern winters. It sprays on like WD-40 but dries to a wax-like finish. Water just beads right off the areas sprayed and did not block any water drain passages -- it's a really thin coat. When I applied my treatment I rubbed it down and kind of cleaned the underbody. The spots I missed around the front suspension have surface rust but everything else I sprayed is rust free. The black factory paint on the suspension components is poor, IMHO, but the MPHD treated areas look new. It was $10/can from a rep and one can did the whole underbody of my Grand and had plenty left over. The only negative, short term like 3-4 days, is that it smells like crayons but after that, no smell.