How do I check rust on a used diesel truck?

Rust can matter more than mileage on an older diesel truck. Surface rust on brackets and exposed metal is common in many climates, but structural rust is different. Inspect the frame rails, crossmembers, cab mounts, bed supports, rocker panels, wheel arches, brake and fuel lines, spring hangers, hitch area, and suspension mounting points. Look for swelling, flaking scale, holes, fresh undercoating over dirty metal, and mismatched repairs. Ask for underside photos before traveling. A truck with a clean drivetrain but serious frame rust can become a bad buy fast. If you are unsure, pay for an inspection before committing.