They salt the crap out of the roads here in Wisconsin. Every spring and fall I'll wash the undersides really well and then touch up any rust I find with rust preventer. Then a week later go back under the vehicle and spray fluid film every where I can including removing tail lights and spraying behind them. Not much behind them on the suburban but can cover quite a bit of the inside bed side on my pickup.
The fluid film isn't the greatest (waxy/greasy) if you have to do any work, suspension or what have you under the vehicle, but it sure beats the hell out of having to fight rusted nuts and bolts.
Agreed 100% with
@15burban. I live in WI as well and do the same thing. The trick is to start with a clean and DRY vehicle before putting any coating on. After 20+ years of perfecting my system, I only go with fluid film (or equivalent).
Do NOT use rubberized undercoating. It will only work if you put it on something completely new, and you have to coat every inch so there's no missed spots. Once it dries, it can shrink and Crack, therefore allowing moisture to get behind it and rust without you seeing it until it is a BIG issue. If you want to touch up areas with this where you cleaned rust off, that works, but make sure to cover it again with Fluid Film.
Buy a professional rust proofing gun for around $250. It has hoses and attachments to get inside doors, frames, rocker panels, etc. 2 gallons of fluid film for about $100 and you're all set. Fluid film can/will wash off the areas exposed to high pressure spray and will need to be touched up before next season. I touch up both Denali's in Oct/Nov and then don't worry about them until spring.
Another thing to note: unless your vehicle will be constantly "wet" from being covered in salt (ie: not drying off in a garage, parking on the street or driveway year round, or not cold enough for salt to be inactive), do yourself a favor and skip all the under body flushes. If it's always outside and sweating from salt residue, then give it a light rinse by hand to wash it off.
Using the high pressure under carriage wash just blows salt where it would never get otherwise. If you have the entire underside and rockers coated with fluid film, then just leave it be, it will do the work for you. Also, around here, car washes are required to reuse a certain percentage of water - nothing like putting extra salt everywhere under pressure. I stay away from those and use my hose.
Long winded, but this is a system that works. I still live in the midwest, but my vehicles haven't looked like it in many years and many, many miles.
Best of luck on the crummy change to snow remediation.