In your area rust. Around the bottoms of the doors, windshield, and I think lower quarter panels.
Drivetrains on these cars are very good even with high miles but there are gremlins.
Bad electrical grounds, checked headers, poor brakes, poor paint, and others I am sure. But, these cars are worth the troubles.
Also more and more parts are getting harder to come by.
If it's an 86-87 car your after then make sure it has a code (7) engine, eigth digit of the VIN. Also open the trunk and verify on the RPO lable on the inside of the trunk lid that the car has RPO code (WE2) that's the option code for "Grand National" If the RPO lable is missing then thats a good sign the car has been in a bad accident. Verify that the VIN on the RPO lable matches the car.
And as posted by ES, look for rust on the bottom of the doors or for signs of repair. These cars also rust around the rear wheel wells and inside the rear wheel wells into the trunk. And make sure to look at the frame, if these cars are driven in the winter for a number of years the rear frame extensions will rust out.
Good luck, and happy hunting
Manitoba Grand National Association