Rustproof your cars for $10 -$12 a year.
I keep my cars for about 5 years, I do the rustproofing myself. Just go and buy a can of Rustcheck at Canadian Tire. Spray it into the doors from the drain holes at the bottom of the doors, that is where cars rust the most. Spray up and away towards the outside of the car to get the inside of the door panel. With new cars very often you will have access to the inside of the front fender from the engine compartment. The rear wheel wells of newer cars look like they are rubber coated and do not need much rust proofing. The rustproof places use the same oil, and just to keep you satiusfied they overspray so that when you see all that oil dripping out of your car you will think you got a good deal. The truth is that most of their cost is in labor and overhead.
I suspect that doing it a little at a time and doing it at the right time will protect your car much better than all at once. Due to the heat last week, I noticed a lot of oil at the bottom of the doors going up on the outside. So I will do a quick spray in autum and early winter. I spray the center hubs of my wheels in the winter, my hubs are not rusty like other cars.