do it new, not later, if your gonna do it at all.
and it's 50/50 at best on the benifit, or nonbenifit of doing it.
powerwash undersides, or give a good rinse....
(altho.. mine rusts just as quick as a couple of guys I know who just drive 'em, and ignore 'em, so I am beginning to question.. all the rinsing)
Old timey method...
save your drain oil.. thin it a bit with kerosene.. get an air nozzle with a pick up tube.. suit up in some old coveralls. and get a resperator..
put the pick up tube in the oil/kero mix.. and spray the heck out of the undersides (hit the inside of the doors, and tailgate too)... then go drive around till the drippin's done

, or even better whip it down a dirt road , and let the dust and dirt pack the oil mix onto the undersides...
do this 2-3 times a year...
don't tell any environatzis of this method

..
have not done this in years..but, I had an old narrow gage bronco for years. and used to do this.. that style bronco was known to rot out.. mine didn't.. at least as fast as the others down here.... had it for over a decade on the beach.. and of all the ones I've owned, that had the most beach time & miles on it, and least rot.
buggy never had a squeak or a ratlle in it either
