I have used a product called "Fluid Film" that contains lanolin for a
couple of years with good results.  It is made by Eureka Chemical
Company, 234 Lawrence Ave., PO Box2205 South San Francisco, CA 94080.  I
get it at my local John Deere dealer, either in spray cans (16 oz) or
gallon cans.  I have used it on farm equipment to prevent rust on
unpainted surfaces during the storage periods.

On my Vanagon(s) I make a diluted blend, mixing it 50/50 with WD-40, and
spray the underside and seams using a "parts cleaner" venturi siphon
from Harbor Freight and compressed air.  Since this stuff doesn't cause
rubber to rot, you can just blast the whole underside without worrying
about damaging bushings, covers, etc. that are made of rubber.  Doing a
van only takes about a pint of each product.  Where I have used this
stuff there has been no new rust. It penetrates crevices well when
thinned with WD-40 and stays on fairly well through the season.  If it's
a daily driver, twice a winter would probably be worthwhile.

It has a "baa-ed" odor to it that you'll recognize if you've been around
sheep, but it's not all that offensive.

Chuck Hill