Rust on a used car: what is cosmetic and what fails

Not all rust is equal. Surface rust on a wing or a door edge is usually cosmetic and will not fail an MOT. Corrosion in the structure near the parts that hold the car together, such as suspension mounts, seat belt anchorages and brake components, is different. The DVSA inspection manual calls these prescribed areas, and it fails a car when corrosion there has made a hole or the metal no longer feels firm. Brake pipes have their own rules. For a buyer, the question is where the rust is, not how much of it you can see.

Why location matters more than size

The DVSA inspection manual makes the point clearly in Appendix A. A small amount of corrosion in an important part of the structure can make a car unsafe. A lot of corrosion on a part that carries no load may have little or no effect on safety.

So a rusty bubble on a rear wing can look worse than a small patch under the car, but the patch under the car may matter far more.

What the manual calls prescribed areas

Prescribed areas are the parts of the structure that matter most for safety. The manual defines them as:

• load-bearing parts of the car to which testable items are mounted, such as parts of the brakes, steering, suspension and seat belts;
• any load-bearing or supporting structure or panelling within 30cm of those mounting points.

The manual gives an example. When a tester looks at a seat belt mounting on an inner sill, they must also consider the outer sill, the door pillar, the floor panel and any other structural part within 30cm of the mounting point.

That is why sills matter so much on many cars. They are often load-bearing and sit close to seat belt and suspension mountings.

When corrosion in a prescribed area fails

The manual tells testers to reject corrosion in a prescribed area if:

1. the corrosion has caused a hole in the metal;
2. the area does not feel firm when pressed with finger and thumb;
3. finger and thumb pressure, or the tester's corrosion assessment tool, makes a hole.

Testers use their eyes, finger and thumb pressure and, where needed, a light tap or scrape with the tool. The manual says the tool must not be used for heavy scraping or poking.

Corrosion outside prescribed areas

Section 6 of the manual covers the general structure outside prescribed areas. Here, a tester should only fail the car if the corrosion means the braking or steering is affected by misalignment, or the strength or continuity of the overall structure is significantly reduced. A main load-bearing member fractured or deformed so that steering or braking is likely to be affected is a dangerous defect.

Bodywork has one more rule. A body panel that is damaged or corroded and likely to cause injury when someone brushes against it, or that is insecure, is a major defect. So a sharp, rusted-through edge can fail even if it carries no load.

Brake pipes and highly stressed parts

Brake pipes are checked under the brakes section, not the structure section. Excessively corroded brake pipes are a major defect, and a pipe at imminent risk of failing is dangerous. Our guide to brake pipe corrosion on an MOT covers this in detail.

The manual also singles out highly stressed components, such as steering and suspension arms. These fail if corrosion has seriously reduced the thickness of the metal or caused a hole or split. Thin pressed-steel parts, such as some subframes and suspension mountings, can rust badly in small, hard-to-see spots.

How repairs are judged

A welded repair can pass if it is done properly. The manual says structural repairs must appear as strong as the original. Patch repairs normally need to be continuously seam welded.

It also lists repairs that are not acceptable on load-bearing members, including:

• gas brazing;
• soldering;
• adhesive bonding;
• fibre reinforcement;
• body filler.

So filler over a rusted sill is not a repair in MOT terms. Fresh underseal or thick paint in one area can hide a poor repair, and the manual accepts that testers cannot always see the method. If in doubt, they pass and advise.

What to check at a viewing

Take a torch and something to kneel on. You do not need to crawl under the car, but look as far as you safely can.

• Sills. Run along both sides below the doors. Look for bubbling paint, holes, flaking metal and fresh filler or underseal in one patch.
• Wheel arches. Look inside each arch, especially at the rear, and at the lip of the arch.
• Jacking points. These are often near the ends of the sills. Crushed or rusted jacking points can hide weak metal behind.
• Subframes and suspension mounts. Look behind the front and rear wheels for heavy flaking rust on the frames and brackets that hold the suspension.
• Floor and boot floor. Lift the boot carpet and check the spare wheel well for water, rust or holes.
• Door bottoms and tailgate. Usually cosmetic, but worth noting for the price.
• Brake pipes. Look along the underside near the rear wheels for heavily rusted pipes.

Press gently. Metal that flexes or crumbles under your thumb is a bad sign. Do not damage the seller's car, though. If you are unsure, see the car and ask a mechanic to put it on a ramp.

Reading corrosion in the MOT history

The MOT history is one of the most useful tools for rust, because corrosion rarely appears overnight. GOV.UK's MOT history service shows past fails and advisories. Look for:

• Repeat advisories that mention corrosion in the same area year after year. The rust is still there and getting worse.
• Advisories that become fails. Ask what was repaired and see the invoice.
• Words such as "sill", "subframe", "suspension mounting" or "seat belt anchorage" next to corrosion. These point to structure, not cosmetics.
• Rust on "underside" or "underbody" generally. This tells you the tester saw enough to mention.

Our MOT advisory checker explains common advisory wording, and our guide to reading MOT history explains the pattern to look for. Our MOT fail rate pages show how often each model fails by age in Great Britain.

Using rust to negotiate

Cosmetic rust is a fair point to raise on price, but it rarely stops a sale. Structural corrosion is different. If the MOT history shows a growing corrosion advisory on a sill or subframe, get a written quote for the repair before you make an offer, or ask the seller to have it done and show you the invoice.

If you find a hole in a sill or a crumbling suspension mount, the car may not pass its next MOT. Our guide to buying a car that failed its MOT covers what that means.

Frequently asked questions

Will surface rust fail an MOT?

Usually not. Rust on panels that carry no load is generally cosmetic. It can fail if it leaves a sharp edge likely to cause injury, or if it is in a prescribed area, such as within 30cm of a seat belt or suspension mounting, and has made a hole or left the metal soft.

What is the 30cm rule for MOT corrosion?

The manual treats load-bearing structure within 30cm of the mounting point of a testable item, such as a seat belt or suspension part, as a prescribed area. Corrosion there fails if it has made a hole or the metal does not feel firm.

Does a rusty sill fail an MOT?

It can. Sills are often load-bearing and close to seat belt and suspension mountings. If corrosion there has made a hole or the metal is soft, it fails.

Can rust be repaired with filler to pass an MOT?

No. The manual does not accept body filler, adhesive bonding, fibre reinforcement, soldering or gas brazing as repairs to load-bearing members. Structural repairs are normally welded.