Ok, this is the observations of a Montrealer.
Cars rust here before your eyes while waiting for the traffic light to change.
Mazda: The 2010 Mazda 3 Hatches seem to be holding up ok.
Prior years and recent stuff is very hit and miss.
I had a buddy I worked with say he had to replace all 4 doors due to rust on his Mazda 3. I don't remember the year, but it was 3 years old at the time. I think it was a 2008 or somewhere thereabout.
My girlfriends 2010 3 Sport hatch is holding up on the sheet metal well, but underneath it is starting to look bad.
She only has ~6,000 km on the car as an ultra low mileage vehicle. She is a nervous driver so we always use my Fit. Also the Fit seems more open as we have a 4 year old daughter still in a car seat.
But in traffic, I see some 2009 Mazda 3 sedans with rust pouring out of every opening in the sheet metal.
Around the side mirrors, trunk, all the door handles etc.
The theory or speculation I heard at school is that Mazda tried to use very low nickel recycled sheet metal and hoped that the zinc and paint would keep corrosion away.
Well, it didn't work very well.
The older cars up here that we see that have the least rust are the Toyotas.
You can see old white boxy Toyotas without a spec of rust up here still.
Nervous Mazda owners get their cars undercoated every year to the point that all the bushings, boots and seals need replacement as they disintegrate from the oil.
Hondas start to really rust after the 10 year mark up here, and people keep them because the engines really hold up even if the burn oil.