Rust: where to look depends on how the car is built

Almost every rust guide gives you the same list of places to look: wheel arches, sills, door bottoms, boot floor. The list is not wrong, but it is the same list for every car, and it leaves out the question that decides whether what you found is cosmetic or terminal.

That question is how the car is built. On one kind of car the body is the structure, and a hole in the wrong panel is a structural repair. On the other the body sits on a frame, and the same hole is a panel you can replace. Same rust, completely different purchase.

Which one you are looking at

Body-on-frame. A ladder frame — two rails down the length of the car, joined by crossmembers — carries the suspension, drivetrain and loads. The body bolts on top through rubber or metal mounts. Almost everything pre-war, most American full-size cars into the 1960s and 70s, most trucks and large SUVs, and most British sports cars with separate chassis.

Unibody (monocoque). The body shell and floor are the structure. There is no separate frame; instead there are box sections, rails pressed into the floor, inner wings, strut or shock towers and sills doing the frame's job while also being the thing you can see. Most cars from the 1960s onward, nearly everything European and Japanese, and most modern collectibles.

Hybrids exist and they are the traps. Subframes bolted to a unibody shell at the front, or a unibody tub with a bolt-on front structure. The mounting points are then the critical areas, because they are where suspension loads enter sheet metal.

If you are not certain, look underneath for continuous rails running the full length under the doors and beyond the wheels. Full-length rails with the body sitting above them: body-on-frame. Floor pressings and no separate rails: unibody.

On a body-on-frame car

Inspect the frame first and the body second. This is the reverse of the instinct, because the body is what you can see and what the photographs show.

Look at:

• The frame rails, along their whole length, with a torch. Rust inside a boxed rail works outward, so the exterior can look passable while the section is gone. Tap along it; a dull note where the rest rings is a soft section.
• Crossmembers and outriggers, especially where they meet the rails — welded joints trap moisture and are where cracks start.
• Body mounts. These are a defined failure item: a mount that is broken, cracked, badly deteriorated or missing is a fail on a safety inspection in some jurisdictions, and a car with collapsed mounts will have doors and panel gaps that never line up again until it is fixed.
• Suspension and steering pickup points, and the area around the spring hangers and shackles.
• Repair history. Plated-over frame sections, fresh undercoat on a car with no other fresh work, and seam sealer applied over rust rather than over clean metal.

And then: body rust on these cars is mostly cosmetic and financial, not structural. Fenders, doors, rockers and floors can be bad and the car remains a car. That is a repair bill, which is a different kind of problem from a frame.

On a unibody car

The panels are the frame. There is no second structure to fall back on, so the question is not "is this panel rusty" but "is this panel load-bearing".

Look at:

• Sills / rocker panels. On a unibody these are structural, and they are the most commonly bodged area on the entire car because the outer skin is easy to replace and the inner structure behind it is not. An outer sill welded over a rotten inner sill looks fine and carries nothing. Press hard underneath; check for filler with a magnet.
• Strut and shock towers. Suspension loads land here and the structure flexes in normal use, so rust plus cycling is how cracks start. Open the bonnet and look at the top of the tower and the seam where it meets the inner wing.
• Floor pans, and specifically where they meet the sills and the transmission tunnel — not the middle of the pan, which is easy to patch, but the edges where it ties into everything else.
• A, B and C pillars and their bases, and the screen surrounds. A rotten lower A-pillar is a serious repair on a unibody and a nasty one on a convertible, which has no roof helping it.
• Suspension mounting points, front and rear subframe mounts, and the rear spring or trailing-arm mounts.
• Boot floor, spare well and rear chassis legs, which on many cars are the rearmost structural members.
• Jacking points. Cheap to ignore, and a reliable proxy for the state of the structure next to them.

Panel gaps and door fit tell you about the shell. A door that drops or needs lifting, gaps that taper, a boot lid that sits proud on one side — on a unibody these are structural symptoms, not adjustment problems.

What to bring and what it tells you

A torch, a small mirror on a telescoping handle, a magnet, and something to lie on. Overalls, honestly.

The magnet finds filler — it will not stick to it at all. The mirror gets you the back of the sill and the top of the frame rail. The torch is for inside the rails and the cavities, which is where the answer usually is.

Two things worth knowing about what you find. Surface rust on the underside of a fifty-year-old car is normal and tells you very little; scaling, flaking and anything that gives under a thumb is different in kind, not degree. And fresh underseal on an otherwise unrestored car is a question, not a feature — it is the standard way of making a floor photograph well.

What this means when you are buying

Decide which structure the car has before you decide what the rust means. The same photograph of a rotten rocker is a bill on a body-on-frame car and a structural rebuild on a unibody, and those are not close to the same purchase.

Where the rust is also tells you where the car lived and how it was stored, and that generalises to everything you have not looked at yet. See the pre-purchase inspection, era by era for what an inspector should be covering beyond the structure, and condition grades for how this feeds into what the car is actually described as.