Years of road salt, wet winters or coastal air can rust a car from underneath long before it looks old. Rust is a wear problem. Many service contracts treat it as outside the breakdown they cover, so the job is to find it early, name where it is, and check the factory corrosion term before you sign anything.
Where rust starts
- Frame rails and the rear subframe mounts.
- Rocker panels and the sills under the doors.
- Wheel arches and the edges of the floor pan under the mats.
- Brake lines and fuel lines where they run along the underside.
How to tell it has started
Look for bubbling paint on a panel edge. Press the sill or floor and feel for give. Scrape gently and note flakes that come away. Check brake lines for orange color or wetness at the joints. Smell near the tank for fuel. Any of these means someone should look under the car with a good light before a contract starts.
Read the corrosion term
Many factory warranties carry a separate corrosion or rust term with its own years and miles limits. It is usually in the owner guide or the warranty booklet, not in the service contract. Find it. Note the years and miles it names, and whether it covers perforation only or surface rust as well. The page on printed limits at one hundred fifty thousand miles shows where stated limits start to matter for older cars.
What to do with what you find
Write down where you found the rust, when you saw it, and the condition around it. Take clear photos. Ask the seller to state in writing whether the contract excludes that area. Arrange an inspection by a shop of your choice before any contract starts, and keep the shop's notes with the contract file.