How to spot hidden rust on a used car

Why rust matters more than most London buyers realise

London is quietly brutal on car bodies. Short stop-start journeys mean the car never gets hot enough to dry out properly, and condensation sits inside the panels for weeks. Add street parking under trees, leaf litter blocking the scuttle drains, and the damp, chilly months where nothing seems to dry, and you have ideal conditions for corrosion.

A car that has spent its life doing suburban supermarket runs can be far rustier than one that has covered motorway miles. Low mileage sounds lovely until you realise it often means years of standing still in a damp courtyard, which is exactly how sills rot from the inside out.

Rust also happens to be the cheapest thing for a seller to disguise and one of the most expensive things for you to put right. MOT testers are relaxed about scruffy cosmetics but strict about structure — sills, floor pans, suspension mountings and seatbelt anchorages. The difference between a small welding job and a car that is beyond saving is often a few inches of bubbling paint you did not bend down to check.

Turn up prepared, and pick your moment

You do not need much, but you do need it with you. Turning up empty-handed means you will talk yourself into believing the paint looks fine.

• A bright torch or your phone light — you will be looking into shadows.
• A small magnet, ideally a weak fridge magnet.
• An old rag and a pair of gloves.
• A small flat screwdriver or plastic scraper, to be used only with the owner's permission.

View in daylight, and ideally after a spell of rain rather than during a downpour. Water leaves tell-tale tracks where it has been running and pooling. Avoid dark evenings and cramped underground car parks where you cannot open the doors fully or kneel beside the car without being watched impatiently. Give yourself twenty minutes and do not let anyone hurry you around the car.

Start low: sills, arches and the underside edges

Get down on your hands and knees. That is where the honest information lives. Run your fingers along the full length of the outer sill on both sides, feeling for bubbling, roughness, or paint with an orange-peel texture. Pay particular attention where the sill meets the rear wheel arch — that joint is the first place to go on most cars.

Rear wheel arches usually rot before the fronts, especially along the inner lip. Feel up inside the arch behind any plastic liner, then check the outer lip and the seam where the bumper meets the wing. A common trick is to fill and repaint an arch lip, so look for paint that is a slightly different shade, or a suspiciously crisp edge where the repair was blended.

While you are down there, check the jacking points. These get crushed by years of careless tyre fitters and each dent becomes a rust trap. Have a look at the boot floor and spare wheel well too — standing water there tells you the seals above are letting it in.

Door seams, boot edges and under the bonnet

Open every door and look along the bottom seam, where the outer skin folds over the frame. Shine the torch at a shallow angle along the seam; bubbling shows up far better in raking light. Check the top of the door frame where the rubber seal sits, and the gutter rail above the doors.

Under the bonnet, the usual culprits are the inner wings, the strut tops, the battery tray, and the seams where the front wing bolts to the inner wing. The front crossmember and radiator support panel often corrode early, and while they are cheap enough to sort at the first sign of trouble, they become a serious job once they are holed. Look behind the headlights and around the washer bottle as well.

Finally, remember that rust is often a symptom. Failed windscreen or boot seals let water in, and the damage shows up somewhere else entirely.

The three tests that separate surface rust from real rot

• Look at the paint edge. Bubbling means corrosion is pushing the paint out from underneath. By the time you can see a bubble, the rot has already started beneath it.
• The magnet test. A magnet sticks to steel but not to filler. If it slides off a sill or arch, or clings only weakly, ask what is underneath.
• The tap test. Tap along the panel with your knuckle. A crisp ring suggests sound metal; a dull thud with a faint crunch usually means filler, sound deadening or laminated rust. A sudden change in tone as you move along a panel is the giveaway.

Only probe with a screwdriver with permission, and gently. You are checking whether flakes come away, not making fresh holes. If the metal crumbles, that panel is finished. Be suspicious of thick, glossy, freshly applied underseal across the sills and arches — it is a classic way to hide an active problem. Ask when it was applied and why.

What to walk away from — and what to price in

Light surface rust on a bolt-on panel such as a wing or a door is manageable and often worth living with. Walk away from rotten sills, holed floor pans, crumbling subframe or suspension mountings, corroded seatbelt anchorages, and anything that has been plated with overlapping patches, which tends to be a repeat problem year after year.

If you do decide to proceed, price the work honestly. A proper sill repair runs into hundreds per side, and once a specialist starts cutting, they usually find more. Use what you have found as a negotiating point rather than a reason to be emotional: "Both rear arches are bubbling and the offside sill has been filled — I would want a body shop to look at it before I commit."

Take a friend, take photos, and take your time. If anything about the structure worries you, walk away. There are plenty of cars in London, and the rusty one will still be rusty when you come to sell it.