Hi, fellow Canuck here.
Very frustrating to see the rust. One of the f'ing joys of living where they use salt is rust.
Rustolium, Tremclad's anti-rust paint, the POR product mentioned above, Ziebart (in a spray bomb, $17.00 per can) any anti-rust paint or a gallon of roofing tar (Bulldog) are the only products that will help now.
The stuff that the dealers use (just about all of them) is at best worthless, but a wonderful revenue generator for the dealer.
As the guys have suggested, before the really cold weather hits, find a dry day, crawl underneath and spray the entire bottom (body and frame) with the anti-rust paint, Ziebart or product of choice. If you want to really do a good job, use the Bulldog roofing tar, a paint brush, cloths you never want to see again and coat every thing in sight except for the exhaust system, cat. converter, and braking system.
For the muffler (made of relatively low grade stainless steel), sand it if you want, but I used a high temperature manifold spray paint (black) and painted it. One it made that huge silver thing seem to disappear, and two it protects it from the rust (but only for a couple of years). Rust in our "salt-land" is inevitable.
Hope this helps. Not what you wanted to hear, but...