Mostly not a problem with modern cars (built within the past 20 years, or so). When I was a kid, I often saw cars running in the streets driving around with missing body panels. Older Japanese models were very bad with rust, and a lot of them rusted away before becoming mechanically worn out. Back then this meant you often saw older German or American models with a Japanese engine in it.
Originally Posted by MetroMPG
The rustproofing has been improved significantly since the 90's or so. Our Daihatsu has small spots of surface rust in places but it's not much of a problem. You still see 90's Japanese cars with beaten-up body panels but little rust on them. The salt air isn't so much of an issue, but occasionally there would be floods near the coast with salt water coming into the streets. You have to avoid those.
Skilled labor is fairly cheap, too. If rust becomes a problem just hire a guy to cut away, weld in a new piece, and repaint. But for the most part this is an issue for older cars.
View my fuel log 2012 Mirage GLS 1.2 manual: 33.3 mpg (US) ... 14.1 km/L ... 7.1 L/100 km ... 40.0 mpg (Imp)