For rust proofing I don't think you can beat used engine oil, just spray everything underneath once a year preferably in the summer and give all the access holes in the frame a good soaking as well. When you are done take her for a drive down a dusty road, the dust will stick to the oil and leave a slimy sludge all over, this will repel all the moisture. A good sprayer to use is the garden variety with a pump up bottle, if it don't want to spray properly as is, mix diesel fuel with the oil to thin it down a little.
I have noticed that vehicles that are garaged overnight tend to rust more that those left outside, I think this is due to lack of a breeze, the hot parts just steam the vehicle if put inside while wet.
PS I come from the UK where the weather is much the same as Vancouver, most vehicles there don't make it past 10 yrs old before a visit to the wreckers yard because of the vast amount of salt they put on the road. The undersides of mine were always rust free, dirty, but rust free.