How to check a car for rust

Safety: only do jobs you are confident with. Never work under a car supported only by a jack, and if a repair affects brakes, steering or suspension and you are unsure, have a garage do it or check your work.

Rust is the one MOT failure that can make an otherwise good car uneconomic to repair. In 2025, 4.1% of car MOT tests failed on body, chassis or structure, and corrosion near suspension and seat-belt mounting points is the most serious kind. Salted winter roads make UK cars particularly prone to it.

Surface rust vs structural rust

• Surface rust: orange discolouration on the surface of the metal, often on exhaust parts, brake discs or suspension arms. Mostly cosmetic, and on its own it doesn’t fail the MOT.
• Structural corrosion: metal that has rotted through or is flaking in layers, within 30cm of a load-bearing area such as suspension mountings, seat-belt anchorages, sills or subframe mounts. This is an MOT failure and needs welding.

A useful test: press firmly with a screwdriver handle (not the tip) on a rusty area. If it is solid, it is surface rust. If it flexes, crunches or gives way, it is structural. Do this gently on a car you don’t own, and ask first.

Where to look

1. Sills: the strip of bodywork below the doors. Open each door and look along the top, then get down and look underneath. Check the jacking points, which take the weight when the car is lifted.
2. Wheel arches: run your fingers up inside the lip of each arch. Bubbling paint here usually means rust underneath.
3. Suspension mounting points: look around the tops of the front struts under the bonnet, and where the rear suspension attaches underneath.
4. Subframes: the frames the engine and suspension bolt onto. Look for heavy flaking where they meet the body.
5. Floor pans and the spare-wheel well: lift the boot carpet and look for water and rust. Wet carpets point to a leak.
6. Door bottoms and the boot lid: drain holes block and trap water.
7. Brake and fuel pipes: corroded brake pipes are an MOT failure and a safety issue.

Warning signs on a used car

• Fresh underseal sprayed thickly over the sills or floor, which can hide rot
• Filler that sounds dull when tapped, or a magnet that won’t stick to a steel panel
• “Corrosion” advisories that repeat at every MOT in the MOT history. The tester has been watching it get worse.

Treating surface rust yourself

1. Clean the area thoroughly and let it dry.
2. Remove loose rust with a wire brush or abrasive pad until you reach solid metal. Wear eye protection and a dust mask.
3. Apply a rust converter or rust-treatment primer, which turns remaining rust into a stable layer.
4. Prime and paint on bodywork, or on underbody parts apply a wax-based underbody protection that stays flexible.
5. On older cars, injecting a cavity wax into the sills and box sections each autumn slows rust from the inside.

Structural rust can’t be fixed with paint, filler or underseal: the rotten metal must be cut out and new steel welded in. Covering structural rot is a common trick, and testers are trained to look for it.

Which cars fail on body and structure most?

From our analysis of every car MOT in 2025, these popular models failed on body and structure most often. If you own or are buying one, it is worth checking this area more carefully. Older cars naturally fail more, so the figures partly reflect each model’s age.

See every model in our MOT data by model.

If you are buying an older car, a pre-purchase inspection or a look on a garage ramp is the only way to be sure of the underside. Our MOT history red flags guide explains which corrosion advisories are serious.