Why this is THE Michigan inspection
Road salt is why two identical cars — same year, same miles — can be a fine buy in one state and scrap in another. In Metro Detroit, every used car has lived through salted winters, so the question is never 'does it have rust?' It's 'where, and how deep?' You can answer that with a flashlight and ten minutes on your knees.
Cosmetic vs structural — the only distinction that matters
Surface rust on a control arm, some scale on the exhaust, brown dust on brackets: normal, and priced in. Rust that FLAKES in layers, holes you can push a screwdriver through, or crumbling edges on the parts that hold the car together: walk away, at any price.
The line is simple — paint and panels are cosmetic; frame, subframe, brake and fuel lines are structural.
The five places to point the flashlight
• ROCKER PANELS — the sills under the doors. Press on them; solid metal doesn't give.
• FRAME RAILS / SUBFRAME — follow them front to back. Layered flaking here is the deal-ender.
• BRAKE AND FUEL LINES — the steel lines running the car's length. A weeping, scabby brake line is a real safety item and a real repair bill.
• STRUT TOWERS — open the hood and trunk, look at the metal around the suspension mounts.
• EXHAUST HANGERS AND FLOOR PANS — pull the carpet edge in the trunk if you can.
Tells that someone's hiding something
Fresh black undercoating on a ten-year-old car is the classic — undercoating is fine as prevention, suspicious as a fresh coat at sale time. Bubbling paint at the bottom of doors and around wheel arches means rust working from the inside out; what you see is the smaller half.
Use it as a negotiation tool, not just a veto
Cosmetic rust you can see is leverage: brake lines that will need doing in a year, rockers that will bubble by spring — these are real, quotable costs, and an honest seller knows it. Bring them up plainly and most Metro Detroit sellers will talk. Every listing on the board connects you straight to the seller, so this conversation happens by text before you ever drive out.