I have a solution to your problem, although most people think I'm crazy for doing it. Having lived in rust free Colorado I brought back two MINT condition 87 4X4's to NH, the rust capitol of th world. I wanted to keep them from rotting out, so I did a little poking around here and sure enough I found a guy who makes a living doing this here. The answer, oil, and lots of it. It works awsome too.

I spray the undercarrage of my vehicles with oil, also treat the inside of my fenders, doors and tailgate. I have tried several different methods and oil mixtures, and have settled on this for a solution. Here is what I find works best. I go to Walmart and buy in bulk straight weight gear oil, ~25 bucks per 5 gallons. I then super heat the oil up on a camping stove to get it nice and watery. I then dump this into a stainless steel hand pump sprayer for bug poision. Pump it up and spray away. After application I drive the truck down a nice dusty road. I do this about twice a year, applying it when I feel it is needed. I'm telling you this works like a champ. I kept both my 87's rust free for 5 years here and one was a regular driver in the winter salt. I have applied this same thing to my 98 T100 and the thing does not have a spec of rust on the undercarrage, doors or tailgate. Now it is messy and smelly and will drip for about 3 to 4 weeks after application, but it beats dealing with rust. Some may find this to be unfriendly to the great outdoors, but the way I look at it, there are those who do far more damage than my truck dripping a little bit of oil on the roads. Typically it takes about 9 quarts to do a complete job, I used about 15 quarts for the first application. For those that live in a part of the country where rust is a problem this is a sure fire way to beat it.